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23.2: Electromagnetic Waves and their Properties

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learning objectives

  • Explain the meaning and importance of Maxwell’s equations

Maxwell’s Equations

Maxwell’s equations are a set of four partial differential equations that, along with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits.

Named after esteemed physicist James Clerk Maxwell, the equations describe the creation and propagation of electric and magnetic fields. Fundamentally, they describe how electric charges and currents create electric and magnetic fields, and how they affect each other.

Maxwell’s equations can be divided into two major subsets. The first two, Gauss’s law and Gauss’s law for magnetism, describe how fields emanate from charges and magnets respectively. The other two, Faraday’s law and Ampere’s law with Maxwell’s correction, describe how induced electric and magnetic fields circulate around their respective sources.

Each of Maxwell’s equations can be looked at from the “microscopic” perspective, which deals with total charge and total current, and the “macroscopic” set, which defines two new auxiliary fields that allow one to perform calculations without knowing microscopic data like atomic-level charges.

Gauss’s Law

Gauss’s law relates an electric field to the charge(s) that create(s) it. The field (E) points towards negative charges and away from positive charges, and from the microscopic perspective, is related to charge density (ρ) and vaccuum permittivity (ε 0 , or permittivity of free space) as:

\[\nabla \cdot \mathbf { E } = \dfrac { \rho } { \epsilon _ { 0 } }\]

Gauss’s Law basically says that a net amount of charge contained within a region of space will generate an electric field that emanates through the surface that surrounds that region.

Example of Gauss’s Law : A positive charge contained within a region of space creates an electric field that emanates from the surface of that region.

Gauss’s Law for Magnetism

Gauss’s law for magnetism states that there are no “magnetic charges (or monopoles)” analogous to electric charges, and that magnetic fields are instead generated by magnetic dipoles . Such dipoles can be represented as loops of current, but in many ways are similar in appearance to positive and negative “magnetic charges” that are inseparable and thus have no formal net “magnetic charge.”

Magnetic field lines form loops such that all field lines that go into an object leave it at some point. Thus, the total magnetic flux through a surface surrounding a magnetic dipole is always zero.

image

Field lines caused by a magnetic dipole : The field lines created by this magnetic dipole either form loops or extend infinitely.

The differential form of Gauss’s law for magnetic for magnetism is

\[\nabla \cdot \mathbf { B } = \mathbf { 0 }\]

Faraday’s Law

Faraday’s law describes how a time-varying magnetic field (or flux) induces an electric field. The principle behind this phenomenon is used in many electric generators. Both macroscopic and microscopic differential equations are the same, relating electric field (E) to the time-partial derivative of magnetic field (B):

\[\nabla \times \mathbf { E } = - \frac { \partial \mathbf { B } } { \partial \mathbf { t } }\]

Ampere’s Circuital Law (with Maxwell’s correction)

Ampere’s law originally stated that magnetic field could be created by electrical current. Maxwell added a second source of magnetic fields in his correction: a changing electric field (or flux), which would induce a magnetic field even in the absence of an electrical current. He named the changing electric field “displacement current.”

Maxwell’s correction shows that self-sustaining electromagnetic waves (light) can travel through empty space even in the absence of moving charges or currents, with the electric field component and magnetic field component each continually changing and each perpetuating the other.

Electromagnetic Waves : Electric (red) and magnetic (blue) waves propagate in phase sinusoidally, and perpendicularly to one another.

The microscopic approach to the Maxwell-corrected Ampere’s law relates magnetic field (B) to current density (J, or current per unit cross sectional area) and the time-partial derivative of electric field (E):

\[\nabla \times \mathbf { B } = \mu _ { 0 } \mathbf { J } + \mu _ { 0 } \epsilon _ { 0 } \frac { \partial \mathbf { E } } { \partial t }\]

The Production of Electromagnetic Waves

Electromagnetic waves are the combination of electric and magnetic field waves produced by moving charges.

  • Explain the self-perpetuating behavior of an electromagnetic wave

Electromagnetic waves

Electromagnetic radiation, is a form of energy emitted by moving charged particles. As it travels through space it behaves like a wave, and has an oscillating electric field component and an oscillating magnetic field. These waves oscillate perpendicularly to and in phase with one another.

image

Electromagnetic Wave : Electromagnetic waves are a self-propagating transverse wave of oscillating electric and magnetic fields. The direction of the electric field is indicated in blue, the magnetic field in red, and the wave propagates in the positive x-direction. Notice that the electric and magnetic field waves are in phase.

The creation of all electromagnetic waves begins with a charged particle. This charged particle creates an electric field (which can exert a force on other nearby charged particles). When it accelerates as part of an oscillatory motion, the charged particle creates ripples, or oscillations, in its electric field, and also produces a magnetic field (as predicted by Maxwell’s equations).

Once in motion, the electric and magnetic fields created by a charged particle are self-perpetuating—time-dependent changes in one field (electric or magnetic) produce the other. This means that an electric field that oscillates as a function of time will produce a magnetic field, and a magnetic field that changes as a function of time will produce an electric field. Both electric and magnetic fields in an electromagnetic wave will fluctuate in time, one causing the other to change.

Electromagnetic waves are ubiquitous in nature (i.e., light) and used in modern technology—AM and FM radio, cordless and cellular phones, garage door openers, wireless networks, radar, microwave ovens, etc. These and many more such devices use electromagnetic waves to transmit data and signals.

All the above sources of electromagnetic waves use the simple principle of moving charge, which can be easily modeled. Placing a coin in contact with both terminals of a 9-volt battery produces electromagnetic waves that can be detected by bringing the antenna of a radio (tuned to a static-producing station) within a few inches of the point of contact.

Energy and Momentum

Electromagnetic waves have energy and momentum that are both associated with their wavelength and frequency.

  • Relate energy of an electromagnetic wave with the frequency and wavelength

Electromagnetic radiation can essentially be described as photon streams. These photons are strictly defined as massless, but have both energy and surprisingly, given their lack of mass, momentum, which can be calculated from their wave properties.

Waves were poorly understood until the 1900s, when Max Planck and Albert Einstein developed modern corrections to classical theory.

Planck theorized that “black bodies” (thermal radiators) and other forms of electromagnetic radiation existed not as spectra, but in discrete, “quantized” form. In other words, there were only certain energies an electromagnetic wave could have. In his work he developed what is now known as “Planck’s constant,” which is approximately equal to 6.626×10 -34 J·s.

The energy (E) of a photon can be related to its frequency (f) by Planck’s constant (h):

\[\mathrm { E } = \mathrm { hf } = \frac { \mathrm { hc } } { \lambda }\]

The ratio of speed of light (c) to wavelength (λ) can be substituted in place of f to give the same equation to energy in different terms. Note that energy cannot take any value: it can only exist in increments of frequency times Planck’s constant (or Planck’s constant times c divided by wavelength). Energy of a wave is therefore “quantized. ”

image

Wavelength : Wavelength of the sinusoidal function is represented by λ.

Momentum is classically defined as the product of mass and velocity and thus would intuitively seem irrelevant to a discussion of electromagnetic radiation, which is both massless and composed of waves.

However, Einstein proved that light can act as particles in some circumstances, and that a wave-particle duality exists. And, given that he related energy and mass (E=mc 2 ), it becomes more conceivable that a wave (which has an energy value) not only has an equation to mass but a momentum as well.

And indeed, Einstein proved that the momentum (p) of a photon is the ratio of its energy to the speed of light.

\[\mathrm { p } = \dfrac { \mathrm { E } } { \mathrm { c } } = \dfrac { \mathrm { hf } } { \mathrm { c } } = \dfrac { \mathrm { h } } { \lambda }\]

Substituting E with hc/λ cancels the c terms, making momentum also equal to the simple ratio of Planck’s constant to wavelength.

The Speed of Light

The speed of light in a vacuum is one of the most fundamental constant in physics, playing a pivotal role in modern physics.

  • Relate speed of light with the index of refraction of the medium

The speed of light is generally a point of comparison to express that something is fast. shows a scale representation of the time it takes a beam of light to reach the moon from Earth. But what exactly is the speed of light?

Light Going from Earth to the Moon : A beam of light is depicted travelling between the Earth and the Moon in the time it takes a light pulse to move between them: 1.255 seconds at their mean orbital (surface-to-surface) distance. The relative sizes and separation of the Earth–Moon system are shown to scale.

It is just that: the speed of a photon or light particle. The speed of light in a vacuum (commonly written as c) is 299,792,458 meters per second. This is a universal physical constant used in many areas of physics. For example, you might be familiar with the equation:

\[\mathrm { E } = \mathrm { mc } ^ { 2 }\]

where E = Energy and m = mass. This is known as the mass-energy equivalence, and it uses the speed of light to interrelate space and time. This not only explains the energy a body of mass contains, but also explains the hindrance mass has on speed.

There are many uses for the speed of light in a vacuum, such as in special relativity, which says that c is the natural speed limit and nothing can move faster than it. However, we know from our understanding of physics (and previous atoms) that the speed at which something travels also depends on the medium through which it is traveling. The speed at which light propagates through transparent materials (air, glass, etc.,) is dependent on the refractive index of that material, n:

\[\mathrm { v } = \dfrac { \mathrm { c } } { \mathrm { n } }\]

where v = actual velocity of light moving through the medium, c = speed of light in a vacuum, and n = refractive index of medium. The refractive index of air is about 1.0003, and from this equation we can find that the speed of visible light in air is about 90 km/s slower than c.

As mentioned earlier, the speed of light (usually of light in a vacuum) is used in many areas of physics. Below is an example of an application of the constant c.

The Lorentz Factor

Fast-moving objects exhibit some properties that are counterintuitive from the perspective of classical mechanics. For example, length contracts and time dilates (runs slower) for objects in motion. The effects are typically minute, but are noticeable at sufficiently high speeds. The Lorentz factor (γ) is the factor by which length shortens and time dilates as a function of velocity (v):

\[\gamma = \left( 1 - \mathrm{ v } ^ { 2 } / \mathrm { c } ^ { 2 } \right) ^ { - 1 / 2 } \gamma = \left( 1 - \mathrm { v } ^ { 2 } / \mathrm { c } ^ { 2 } \right) ^ { - 1 / 2 } \gamma = \left( 1 - \mathrm { v } ^ { 2 } / \mathrm { c } ^ { 2 } \right) ^ { - 1 / 2 }\]

At low velocities, the quotient of v 2 /c 2 is sufficiently close to 0 such that γ is approximately 1. However, as velocity approaches c, γ increases rapidly towards infinity.

The Doppler Effect

The Doppler Effect is the change in a wave’s perceived frequency that results from the source’s motion, the observer, and the medium.

  • Give examples of daily observations of the Doppler effect

The Doppler effect is a periodic event’s change in frequency for an observer in motion relative to the event’s source. Typically, this periodic event is a wave.

Most people have experienced the Doppler effect in action. Consider an emergency vehicle in motion, sounding its siren. As it approaches an observer, the pitch of the sound (its frequency) sounds higher than it actually is. When the vehicle reaches the observer, the pitch is perceived as it actually is. When the vehicle continues away from the observer, the pitch is perceived as lower than it actually is. From the perspective of an observer inside the vehicle, the pitch of the siren is constant.

The Doppler Effect and Sirens : Waves emitted by a siren in a moving vehicle

The difference in the perceived pitch depending on observer location can be explained by the fact that the siren’s position changes as it emits waves. A wave of sound is emitted by a moving vehicle every millisecond. The vehicle ‘chases’ each wave in one direction. By the time the next wave is emitted, it is closer (relative to an onlooker ahead of the vehicle) to the previous wave than the wave’s frequency would suggest. Relative to an onlooker behind the vehicle, the second wave is further from the first wave than one would expect, which suggests a lower frequency.

The Doppler effect can be caused by any kind of motion. In the example above, the siren moved relative to a stationary observer. If the observer moves relative to the stationary siren, the observer will notice the Doppler effect on the pitch of the siren. Finally, if the medium through which the waves propagate moves, the Doppler effect will be noticed even for a stationary observer. An example of this phenomenon is wind.

Quantitatively, the Doppler effect can be characterized by relating the frequency perceived (f) to the velocity of waves in the medium (c), the velocity of the receiver relative to the medium (v r ), the velocity of the source relative to the medium (v s ), and the actual emitted frequency (f 0 ):

\[\mathrm { f } = \left( \dfrac { \mathrm { c } + \mathrm { v } _ { \mathrm { r } } } { \mathrm { c } + \mathrm { v } _ { \mathrm { s } } } \right) \mathrm { f } _ { 0 }\]

image

The Doppler Effect : Wavelength change due to the motion of source

Momentum Transfer and Radiation Pressure Atom

Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic (EM) radiation.

  • Explain formation of radiation pressure

Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic (EM) radiation. EM radiation (or photon, which is a quantum of light) carries momentum; this momentum is transferred to an object when the radiation is absorbed or reflected. Perhaps one of the most well know examples of the radiation pressure would be comet tails. Haley’s comet is shown in.

image

Halley’s Comet : As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus. The streams of dust and gas thus released form an atmosphere around the comet (called the coma), and the force exerted on the coma by the Sun’s radiation pressure and solar wind cause the formation of an enormous tail that points away from the Sun.

Although radiation pressure can be understood using classical electrodynamics, here we will examine the quantum mechanical argument. From the perspective of quantum theory, light is made of photons: particles with zero mass but which carry energy and – importantly in this argument – momentum. According to special relativity, because photons are devoid of mass, their energy (E) and momentum (p) are related by E=pc.

Now consider a beam of light perpendicularly incident on a surface, and let us assume the beam of light is totally absorbed. The momentum the photons carry is a conserved quantity (i.e., it cannot be destroyed) so it must be transferred to the surface; thus the absorption of the light beam causes the surface to gain momentum. Newton’s Second Law tells us that force equals rate of change of momentum; thus during each second, the surface experiences a force (or pressure, as pressure is force per unit area) due to the momentum the photons transfer to it.

This gives us: pressure = momentum transferred per second per unit area = energy deposited per second per unit area / c = I/c, (where I is the intensity of the beam of light).

Laser Cooling

There are many variations of laser cooling, but they all use radiation pressure to remove energy from atomic gases (and therefore cool the sample). In laser cooling (sometimes called Doppler cooling), the frequency of light is tuned slightly below an electronic transition in the atom. Because light is detuned to the “red” (i.e., at lower frequency) of the transition, the atoms will absorb more photons if they move towards the light source, due to the Doppler effect. Thus if one applies light from two opposite directions, the atoms will always scatter more photons from the laser beam pointing opposite to their direction of motion (typical setups applies three opposing pairs of laser beams as in ).

image

The Magneto Optical Trap : Experimental setup of Magneto Optical Trap (MOT), which uses radiation pressure to cool atomic species. Atoms are slowed down by absorbing (and emitting) photons.

In each scattering event, the atom loses a momentum equal to the momentum of the photon. If the atom (which is now in the excited state) then emits a photon spontaneously, it will be kicked by the same amount of momentum, only in a random direction. Since the initial momentum loss was opposite to the direction of motion (while the subsequent momentum gain was in a random direction), the overall result of the absorption and emission process is to reduce the speed of the atom. If the absorption and emission are repeated many times, the average speed (and therefore the kinetic energy ) of the atom will be reduced. Since the temperature of a group of atoms is a measure of the average random internal kinetic energy, this is equivalent to cooling the atoms. Simple laser cooling setups can produce a cold sample of atomic gases at around 1mK (=10 -3 K) starting from a room temperature gas.

  • Maxwell’s four equations describe how electric charges and currents create electric and magnetic fields, and how they affect each other.
  • Gauss’s law relates an electric field to the charge(s) that create(s) it.
  • Gauss’s law for magnetism states that there are no “magnetic charges” analogous to electric charges, and that magnetic fields are instead generated by magnetic dipoles.
  • Faraday’s law describes how a time-varying magnetic field (or flux ) induces an electric field. The principle behind this phenomenon is used in many electric generators.
  • Ampere ‘s law originally stated that a magnetic field is created by an electrical current. Maxwell added that a changing electric flux can also generate a magnetic field.
  • Electromagnetic waves consist of both electric and magnetic field waves. These waves oscillate in perpendicular planes with respect to each other, and are in phase.
  • The creation of all electromagnetic waves begins with an oscillating charged particle, which creates oscillating electric and magnetic fields.
  • Once in motion, the electric and magnetic fields that a charged particle creates are self-perpetuating: time-dependent changes in one field (electric or magnetic) produce the other.
  • Max Planck proved that energy of a photon (a stream of which is an electromagnetic wave ) is quantized and can exist in multiples of “Planck’s constant” (denoted as h, approximately equal to 6.626×10 -34 J·s).
  • \(\mathrm { E } = \mathrm { hf } = \frac { \mathrm { hc } } { \lambda } \)describes the energy (E) of a photon as a function of frequency (f), or wavelength (λ).
  • \(\mathrm { p } = \frac { \mathrm { E } } { \mathrm { c } } = \frac { \mathrm { hf } } { \mathrm { c } } = \frac { \mathrm { h } } { \lambda }\) describes the momentum (p) of a photon as a function of its energy, frequency, or wavelength.
  • The maximum possible value for the speed of light is that of light in a vacuum, and this speed is used for a constant in many area of physics.
  • c is the symbol used to represent the speed of light in a vacuum, and its value is 299,792,458 meters per second.
  • When light travels through medium, its speed is hindered by the index of refraction of that medium. Its actual speed can be found with: \(v=\frac{c}{n}\).
  • The Doppler effect is very commonly observed in action.
  • The Doppler effect can be observed in the apparent change in pitch of a siren on an emergency vehicle, according to a stationary observer.
  • The observer will notice the Doppler effect on the pitch of the stationary siren when moving relative to its pitch, or if the medium moves when the observer is stationary.
  • Photons carry momentum (p = E/c). When photons are absorbed or reflected on a surface, the surface receives momentum kicks. This momentum transfer leads to radiation pressure.
  • Electromagnetic radiation applies radiation pressure equal to the Intensity (of light beam) divided by c (speed of light).
  • Laser cooling uses radiation pressure to remove energy from atomic gases. The technique can produce cold samples of gases at 1mK or so.
  • differential equation : An equation involving the derivatives of a function.
  • flux : A quantitative description of the transfer of a given vector quantity through a surface. In this context, we refer to the electric flux and magnetic flux.
  • electromagnetic wave : A wave of oscillating electric and magnetic fields.
  • phase : Waves are said to be “in phase” when they begin at the same part (e.g., crest) of their respective cycles.
  • photon : The quantum of light and other electromagnetic energy, regarded as a discrete particle having zero rest mass, no electric charge, and an indefinitely long lifetime.
  • wavelength : The length of a single cycle of a wave, as measured by the distance between one peak or trough of a wave and the next; it is often designated in physics as λ, and corresponds to the velocity of the wave divided by its frequency.
  • frequency : The quotient of the number of times n a periodic phenomenon occurs over the time t in which it occurs: f = n / t.
  • special relativity : A theory that (neglecting the effects of gravity) reconciles the principle of relativity with the observation that the speed of light is constant in all frames of reference.
  • refractive index : The ratio of the speed of light in air or vacuum to that in another medium.
  • doppler effect : Apparent change in frequency of a wave when the observer and the source of the wave move relative to each other.
  • classical electrodynamics : A branch of theoretical physics that studies consequences of the electromagnetic forces between electric charges and currents.

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Introduction to Electromagnetic Waves

Chapter outline.

The beauty of a coral reef, the warm radiance of sunshine, the sting of sunburn, the X-ray revealing a broken bone, even microwave popcorn—all are brought to us by electromagnetic waves . The list of the various types of electromagnetic waves, ranging from radio transmission waves to nuclear gamma-ray ( γ γ -ray) emissions, is interesting in itself.

Even more intriguing is that all of these widely varied phenomena are different manifestations of the same thing—electromagnetic waves. (See Figure 24.2 .) What are electromagnetic waves? How are they created, and how do they travel? How can we understand and organize their widely varying properties? What is their relationship to electric and magnetic effects? These and other questions will be explored.

Misconception Alert: Sound Waves vs. Radio Waves

Many people confuse sound waves with radio waves , one type of electromagnetic (EM) wave. However, sound and radio waves are completely different phenomena. Sound creates pressure variations (waves) in matter, such as air or water, or your eardrum. Conversely, radio waves are electromagnetic waves , like visible light, infrared, ultraviolet, X-rays, and gamma rays. EM waves don’t need a medium in which to propagate; they can travel through a vacuum, such as outer space.

A radio works because sound waves played by the D.J. at the radio station are converted into electromagnetic waves, then encoded and transmitted in the radio-frequency range. The radio in your car receives the radio waves, decodes the information, and uses a speaker to change it back into a sound wave, bringing sweet music to your ears.

Discovering a New Phenomenon

It is worth noting at the outset that the general phenomenon of electromagnetic waves was predicted by theory before it was realized that light is a form of electromagnetic wave. The prediction was made by James Clerk Maxwell in the mid-19th century when he formulated a single theory combining all the electric and magnetic effects known by scientists at that time. “Electromagnetic waves” was the name he gave to the phenomena his theory predicted.

Such a theoretical prediction followed by experimental verification is an indication of the power of science in general, and physics in particular. The underlying connections and unity of physics allow certain great minds to solve puzzles without having all the pieces. The prediction of electromagnetic waves is one of the most spectacular examples of this power. Certain others, such as the prediction of antimatter, will be discussed in later modules.

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Access for free at https://openstax.org/books/college-physics-2e/pages/1-introduction-to-science-and-the-realm-of-physics-physical-quantities-and-units
  • Authors: Paul Peter Urone, Roger Hinrichs
  • Publisher/website: OpenStax
  • Book title: College Physics 2e
  • Publication date: Jul 13, 2022
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Astronomy & Astrophysics 101: Electromagnetic Spectrum

By ESA/Hubble April 24, 2022

Electromagnetic Spectrum

What Is the Electromagnetic Spectrum?

The electromagnetic spectrum is a range of frequencies of electromagnetic radiation. From long to short wavelength, the EM spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays.

Energy travels through space as electromagnetic (EM) waves, which are made up of oscillating electric and magnetic fields. EM waves do not require a material (such as air or water) to move through, hence they can, unlike sound, travel through empty space. All electromagnetic waves travel at the same speed in a vacuum: the speed of light (which is itself an EM wave).

An EM wave, like other waves, is defined by its wavelength, and the range of wavelengths we observe, from extremely long to very short, is referred to as the EM spectrum. The electromagnetic spectrum is loosely divided into divisions based on how the waves behave when they interact with matter, and each division is given a name. Radio waves have the longest wavelengths, followed by microwaves, infrared, visible light, ultraviolet, x-rays, and finally gamma rays, which have the shortest wavelengths. 

Celestial objects such as stars , planets , and galaxies all emit electromagnetic waves at various wavelengths, therefore telescopes are built to be sensitive to different sections of the electromagnetic spectrum. Shorter wavelengths are referred to as ‘bluer,’ while longer wavelengths are referred to as’redder.’ EM radiation in and near the visible region of the spectrum is generally referred to broadly as ‘light.’

We can construct a more complete image of an object’s structure, composition, and behavior by combining observations at different wavelengths than visible wavelengths alone can convey.

For more than three decades, Hubble has studied the Universe using its 2.4-meter primary mirror and its five science instruments. They observe primarily in the ultraviolet and visible parts of the spectrum, but also have some near- infrared capabilities. Hubble observes in different wavelength bands, one band at a time, each providing different information on the object under study. Each of these wavelengths is reproduced in a different color and these are combined to form a composite image that well resembles the true emission from that celestial object.

Multi-Wavelength and Composite Images of NGC 1512

Astronomers have used this set of single-color images, shown around the edge, to construct the color picture (center) of a ring of star clusters surrounding the core of the galaxy NGC 1512. These pictures were taken by the NASA/ESA Hubble Space Telescope’s Faint Object Camera (FOC), Wide Field and Planetary Camera 2 (WFPC2), and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS). Each image represents a specific color or wavelength region of the spectrum, from ultraviolet to near infrared, and shows the wide wavelength region covered by Hubble. Celestial bodies emit light at a variety of wavelengths, anywhere from gamma rays to radio waves. Astronomers chose to study NGC 1512 in these colors to emphasize important details in the ring of young star clusters surrounding the core. Credit: NASA, ESA, Dan Maoz (Tel-Aviv University, Israel, and Columbia University, USA)

By exploring the image above, you can see how astronomers have used a set of single-color images to construct the color picture of a ring of star clusters surrounding the core of the galaxy NGC 1512. Each image represents a specific color or wavelength region of the spectrum, from ultraviolet to near-infrared, and shows the wide wavelength range covered by Hubble. Astronomers chose to study NGC 1512 in these colors to emphasize important details in the ring of young star clusters surrounding the core.

Word Bank Electromagnetic Spectrum

Electromagnetic Spectrum. Credit: ESA/Hubble

Astronomers use multi-wavelength imagery to study details that might not otherwise be present in visible images. For example, a new multiwavelength observation of Jupiter released in 2020 by Hubble in ultraviolet/visible/near-infrared light of Jupiter gave researchers an entirely new view of the giant planet. These observations provided insights into the altitude and distribution of the planet’s haze and particles and showed Jupiter’s ever-changing cloud patterns. The planet’s aurorae are only visible in the ultraviolet; however, the structure of the red spot is well studied at visible wavelengths.

To celebrate the telescope’s 25th anniversary in 2015, Hubble unveiled two new beautiful portraits of the popular Pillars of Creation , revealing how different details can be studied in visible and near-infrared observations. While the visible light captures the multi-colored glow of gas clouds, the infrared image penetrates much of the obscuring dust and gas to uncover countless newborn stars.

We invite you to watch this Hubblecast that explores how Hubble’s observations differ across different wavelengths of the electromagnetic spectrum, and how these observations will be complemented by those of the James Webb Space Telescope .

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1 comment on "astronomy & astrophysics 101: electromagnetic spectrum".

em waves travel through space

“Astronomers have used this set of single-color images, shown around the edge, to construct the color picture (center) of a ring of star clusters surrounding the core of the galaxy NGC 1512.”

Because a computer display device works with just three primary colors (RGB), I would be interested in knowing just how they compressed 7 different images into three colors. Did they use band ratios or weighted averages, or some other technique?

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Electromagnetic Radiation

Claimed by Zoila de Leon (Fall 2023)

  • 1.1 What is a Electromagnetic(EM) Radiation?
  • 1.2 General Properties
  • 1.3 Problem Solving Method and Equations
  • 1.4 Fields Made by Charges and Fields Made by Monopoles
  • 2 The EM Spectrum
  • 3 Waves and Fields
  • 4 A Mathematical Model
  • 5 Connectedness: X-Rays
  • 6.1 Practice Problems (new section by Zoila)
  • 7 References

What is a Electromagnetic(EM) Radiation?

Electromagnetic radiation is a form of energy that is all around us and takes many forms, such as radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays.

Before 1873, electricity and magnetism were thought to be two different forces. However, in 1873, Scottish Physicist James Maxwell developed his famous theory of electromagnetism. There are four main electro magnetic interactions according to Maxwell:

  • The force of attraction or repulsion between electric charges is inversely proportional to the square of the distance between them
  • Magnetic poles come in pairs that attract and repel each other much as electric charges do
  • An electric current in a wire produces a magnetic field whose direction depends on the direction of the current
  • A moving electric field produces a magnetic field, and vice versa

General Properties

The four Maxwell's Equations provide a complete description of possible spatial patterns of electric and magnetic field in space.

  • The Ampere-Maxwell Law
  • Gauss's Law
  • Faraday's Law

Other than Maxwell's Four equations, there are general properties of all electromagnetic radiation:

  • Electromagnetic radiation can travel through empty space. Most other types of waves must travel through some sort of substance. For example, sound waves need either a gas, solid, or liquid to pass through in order to be heard
  • The speed of light is always a constant (3 x 10^8 m/s)
  • Wavelengths are measured between the distances of either crests or troughs. It is usually characterized by the Greek symbol λ (lambda).

Electromagnetic waves are the self-propagating, mutual oscillation of electric and magnetic fields. The propagation of electromagnetic energy is often referred to as radiation. We can also say that the 'pulse' of these moving fields result in radiation (7).

The equation for propagation is E=cB with c being the speed of light. This equation is derived from combining the two equations E=vB and B=u0e0vE, proving that v is equal to 3e8 meters/second.

Problem Solving Method and Equations

To go about solving/analyzing mathematically an electromagnetic field using Maxwell's equations,this is how we proceed (7)

  • Establish the space and time in which the electric and magnetic fields are present
  • Check that Maxwell's equations can be applied in the situation above
  • Check when the charge accelerates, it produces these fields and therefore radiation
  • Show how these fields would interact with matter

The equation of the Radiative Electric Field is: E= 1/(4πe0)*-qa/(c^2r) where a is the acceleration of the particle, c is the speed of light and r is the distance from the original location of the charge to right before the kink. This kink happens on the electric field because of the slight delay when the charge is moved.

Fields Made by Charges and Fields Made by Monopoles

We can differentiate fields made by charges and the ones made by magnetic monopoles. (7) For fields made by charges, when the charge is

  • at rest, E=1/r^2 and B=0
  • constant speed, E=1/r^2 and B=1/r^2
  • accelerating, E=1/r and B=1/r

For fields made by magnetic monopoles, the first point would have E and B switched.

The EM Spectrum

EM spectrum is a span of enormous range of wavelengths and frequencies. The EM spectrum is generally divided into 7 different regions, in order of decreasing wavelength and increasing energy and frequency. It ranges from Gamma rays to Long Radio Waves. Following are the lists of waves:

  • Visible Light
  • Infrared Rays
  • Long radio waves

em waves travel through space

Although all these waves do different things, there is one thing in common : They all travel in waves.

em waves travel through space

Infrared radiation can be released as heat or thermal energy. It can also be bounced back, which is called near infrared because of its similarities with visible light energy. Infrared Radiation is most commonly used in remote sensing as infrared sensors collect thermal energy, providing us with weather conditions.

em waves travel through space

Visible Light is the only part of the electromagnetic spectrum that humans can see with a naked eye. This part of the spectrum includes a range of different colors that all represent a particular wavelength. Rainbows are formed in this way; light passes through matter in which it is absorbed or reflected based on its wavelength. As a result, some colors are reflected more than other, leading to the creation of a rainbow.

em waves travel through space

Waves and Fields

As we learned in class, electric field is produced when an electron is accelerating. Likewise, EM radiation is created when an atomic particle, like an electron, is accelerated by an electric field. The movement like this produces oscillating electric and magnetic fields, which travel at right angles to each other in a bundle of light energy called a photon. Photons travel in a harmonic wave at the fastest speed possible in the universe.

em waves travel through space

Electromagnetic waves are formed when an electric field couples with a magnetic field. Magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave.

A wavelength (in m) is the distance between two consecutive peaks of a wave. Frequency is the number of waves that form in a given length of time. A wavelength and frequency are interrelated. A short wavelength indicates that the frequency will be higher because one cycle can pass in a shorter amount of time. Likewise, a longer wavelength has a lower frequency because each cycle takes longer to complete.

em waves travel through space

Waves can be classified according to their nature:

  • Mechanical waves
  • Electromagnetic waves

Mechanical Waves

Mechanical waves require a medium (matter) to travel through. Examples are sound waves, water waves, ripples in strings or springs.

Water Waves

Sound Waves

Electromagnetic Waves

Electromagnetic waves do not require a medium (matter) to travel through - they can travel through space. Examples are radio waves, visible light, x-rays.

em waves travel through space

Radio Waves

em waves travel through space

Visible Lights

em waves travel through space

A Mathematical Model

The position of the particle is defined by a sine wave:

y = ymaxsin(wt)

Where w is the angular frequency

Amplitude is the distance from the maximum vertical displacement of the wave to the middle of the wave. The Amplitude of the sinusoidal Wave is the height of the peak in the wave measured from the zero line. This measures the magnitude of oscillation of a particular wave. The Amplitude is important because it tells you the intensity or brightness of a wave in comparison with other waves.

The period of the wave is the time between crests in seconds(s).

T = 2pi/w-----(units of seconds)

Frequency is the number of cycles per second, and is expressed as sec-1 or Hertz(Hz). Frequency is directly proportional to energy and can be express as "

E = hv where E is energy, h is Planck's constant ( 6.62607*10^-34J) and v is frequency

f = 1/T f = w/2pi----(Units Hertz)

Wavelength is the distance between crests in meters. Wavelength is equal to the speed of light times frequency. Longer wavelength waves such as radio waves carry low energy; this is why we can listen to the radio without any harmful consequences. Shorter wavelength waves such as x-rays carry higher energy that can be hazardous to our health.

em waves travel through space

Wavelength and Frequency

The speed of light is the multiplication of the wavelength and frequency.

em waves travel through space

This diagram shows all properties of waves:

ENERGY FLUX

Is defined by the following equation:

Connectedness: X-Rays

Electromagnetic Radiation while commonly thought of as only including visible light, radio waves, UV waves, and gamma rays; also include X-rays. In 1895, X-rays were initially discovered by William Roentgen, who accidentally fell upon the most important discovery about his life (Figure 1). Roentgen was already working on cathode rays, and because of a fluorescent glow that occurred during his experiments, covered his experimental apparatus with heavy black paper. However, when he did this, he discovered a glow coming from a screen several feet away. Through many more experiments, he discovered that a new type of energy, not cathode rays, were the cause of the glow. He named them “x-rays” and received the 1901 Nobel Prize in Physics. Roentgen never patented his monumental discovery and as a result, numerous researchers set out to find a multitude of uses and capitalize on his work.

Primarily, people could now view objects that were hidden from plain view (i.e. scanners in airports). While X-rays are now used in 100’s of professions (security, chemistry, art galleries), its most important function is to view bones to determine abnormalities in humans. In fact, one of Roentgen’s first x-rays was of his wife’s hand (Figure 2). X-rays fall under the scope of electromagnetic radiation because, like all E.R. waves, it is comprised of photons. X-rays have wavelengths between 0.01 to 10 nanometers and fall between UV and Gamma Waves on the E.R. spectrum (Figure 3). There are two main methods in which an x-ray may be formed. Both require a vacuum-filled tube called an x-ray tube (Figure 4). With an anode on one end and a cathode on the other, an electric current is applied and a high energy electron is projected from the cathode, through the vacuum, and at the anode. In the characteristic x-ray generation approach, the electron from the cathode collides with an inner shell electron on an atom on the anode (Figure 5). Both of these electrons are ejected from the atom and an outer shell electron takes the place of the inner shell one. Because the outer electron must have a lower energy to fill the inner shell hole, it releases a photon with the equivalent energy of the difference between the two energy levels in the atom. This photon is the x-ray that is used to view objects such as bones.

In the Bremsstrahlung x-ray generation method, the electron from the cathode is slowed as it passes the nucleus of an atom at the anode (Figure 6). As it slows and its path is changed, the loses energy (kinetic energy). This energy is also released as a photon which is subsequently called an x-ray. Depending on the voltage and current of the tube and the material of the anode, different types (as in wavelengths and energy) of x-rays can be produced and each one. However, all X-rays will continue to pass through objects until it reaches a material dense that stops it. However, density of the material required depends on the energy of the x-ray. For example, during a medical x-ray, x-rays of a certain energy will pass through soft tissue (skin, organs, etc) but not through bones. The x-rays that pass through the soft tissue will strike the screen and the absence of the x-rays absorbed by the bones will cause a negative space on the screen. The areas where x-rays do not strike will form the image of the bone. While the principles remain the same, x-ray machines today use incredible sophisticated technology to specify the type of x-ray they want and have greatly increased in accuracy since Roentgen’s initial discovery.

em waves travel through space

  • Information and photographs are pulled from references 1 through 5 cited below*

Already, during the Ancient Greek and Roman times, light was studied as the presence of deflection and refraction were noticed. Electromagnetic radiation of wavelengths in the early 19th century. The discovery of infrared radiation is ascribed to astronomer William Herschel, who published his results in 1800 before the Royal Society of London. Herschel used a glass Triangular prism (optics)|prism to refract light from the Sun and detected invisible rays that caused heating beyond the red part of the spectrum, through an increase in the temperature recorded with a thermometer. These "calorific rays" were later termed infrared.

In 1801, Rohann Ritter, discovered the presence of ultraviolet light using salts. It was known that light could darken some silver halides and while doing so, he realized that the region beyond the violet bar (therefore ultraviolet) was more effective in changing the color of the halides. However,in 1864, while summarizing the theories of his time accumulating into his famous set of Maxwell equations, James Clerk Maxwell managed to deduce the speed of light being around 3e8 meters per second. This was instrumental in creating the rest of the spectrum.

In 1887-1888 Physicist Heinrich Hertz not only tried to measure the velocity and frequency of electromagnetic radiation waves at other parts of the known spectrum of the time, but he was also able to prove that Maxwell's findings were correct. He did this on the microwave radiation as well.

The discovery of X-rays occurred in 1895 by Wilhelm Rontgen when his barium platinocyanide detector screen began to glow under the presence of a discharge that passed through a cathode ray tube although the latter was completely covered. Once he determined its possible use, he tried to look at his wife's hand using this new discovery. However x-ray spectroscopy was not institutionalized until later by Karl Manne Siegbahn.

In 1900, Paul Villard discovered Gamma rays although he initially thought that they were particles similar to alpha and beta particles which were emitted during radiation. These 'particles' were later proven to be part of the electromagnetic spectrum.

Practice Problems (new section by Zoila)

em waves travel through space

1. Elert, Glenn. "X-rays." X-rays – The Physics Hypertextbook. N.p., n.d. Web. 08 Apr. 2017. http://physics.info/x-ray/

2."X-rays." X-rays. N.p., n.d. Web. 08 Apr. 2017. http://www.physics.isu.edu/radinf/xray.htm

3. "Basics of X-ray PhysicsX-ray production." Welcome to Radiology Masterclass. N.p., n.d. Web. 08 Apr. 2017. http://www.radiologymasterclass.co.uk/tutorials/physics/x-ray_physics_production#top_2nd_img

4. "X-Rays." Image: Electromagnetic Spectrum. N.p., n.d. Web. 08 Apr. 2017. https://www.boundless.com/physics/textbooks/boundless-physics-textbook/electromagnetic-waves-23/the-electromagnetic-spectrum-165/x-rays-597-11175/images/electromagnetic-spectrum/

5. "This Month in Physics History." American Physical Society. N.p., n.d. Web. 08 Apr. 2017. https://www.aps.org/publications/apsnews/200111/history.cfm

6. Editors, Spectroscopy. “The Electromagnetic Spectrum: A History.” Spectroscopy Home, 27 Oct. 2017, www.spectroscopyonline.com/electromagnetic-spectrum-history?id=&sk=&date=&&pageID=4.

7. Chabay, Ruth W., and Bruce A. Sherwood. Matter & Interaction II: Electric & Magnetic Interactions, Version 1.2. John Wiley & Sons, 2003.

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Propagation of an Electromagnetic Wave

Electromagnetic waves are waves which can travel through the vacuum of outer space. Mechanical waves, unlike electromagnetic waves, require the presence of a material medium in order to transport their energy from one location to another. Sound waves are examples of mechanical waves while light waves are examples of electromagnetic waves.

Electromagnetic waves are created by the vibration of an electric charge. This vibration creates a wave which has both an electric and a magnetic component. An electromagnetic wave transports its energy through a vacuum at a speed of 3.00 x 10 8 m/s (a speed value commonly represented by the symbol c ). The propagation of an electromagnetic wave through a material medium occurs at a net speed which is less than 3.00 x 10 8 m/s. This is depicted in the animation below.

The mechanism of energy transport through a medium involves the absorption and reemission of the wave energy by the atoms of the material. When an electromagnetic wave impinges upon the atoms of a material, the energy of that wave is absorbed. The absorption of energy causes the electrons within the atoms to undergo vibrations. After a short period of vibrational motion, the vibrating electrons create a new electromagnetic wave with the same frequency as the first electromagnetic wave. While these vibrations occur for only a very short time, they delay the motion of the wave through the medium. Once the energy of the electromagnetic wave is reemitted by an atom, it travels through a small region of space between atoms. Once it reaches the next atom, the electromagnetic wave is absorbed, transformed into electron vibrations and then reemitted as an electromagnetic wave. While the electromagnetic wave will travel at a speed of c (3 x 10 8 m/s) through the vacuum of interatomic space, the absorption and reemission process causes the net speed of the electromagnetic wave to be less than c. This is observed in the animation below.

The actual speed of an electromagnetic wave through a material medium is dependent upon the optical density of that medium. Different materials cause a different amount of delay due to the absorption and reemission process. Furthermore, different materials have their atoms more closely packed and thus the amount of distance between atoms is less. These two factors are dependent upon the nature of the material through which the electromagnetic wave is traveling. As a result, the speed of an electromagnetic wave is dependent upon the material through which it is traveling.

For more information on physical descriptions of waves, visit The Physics Classroom Tutorial . Detailed information is available there on the following topics:

Mechanical vs. Electromagnetic Waves Wavelike Behaviors of Light The EM and Visible Spectra Light Absorption, Reflection, and Transmission Optical Density and Light Speed  

Return to List of Animations

Electricity – Magnetism

How does electromagnetic wave propagation work?

Explore electromagnetic wave propagation, its characteristics, and mechanisms. Dive into the world of physics, from radio to X-rays.

Understanding Electromagnetic Wave Propagation

Electromagnetic (EM) wave propagation is an essential concept in physics that describes how electromagnetic waves travel through space. These waves, which include light, radio, and X-rays, are fundamental to many aspects of our daily life.

Characteristics of Electromagnetic Waves

  • Transverse Nature: EM waves are transverse in nature, meaning they oscillate perpendicular to the direction of energy propagation.
  • Speed: They all travel at the speed of light in vacuum, approximately 3 x 10 8 meters per second.
  • Wavelength and Frequency: Each type of EM wave is characterized by a specific wavelength and frequency, which determines its position in the electromagnetic spectrum.

Propagation Mechanisms

Electromagnetic wave propagation involves the transmission of energy through a medium or through a vacuum. This process can occur in various ways, depending on the type of wave and the medium through which it is passing.

  • Ground Wave Propagation: This involves the propagation of waves along the surface of the Earth. It is particularly relevant for low-frequency signals.
  • Skywave Propagation: In this method, waves are reflected off the ionosphere back to Earth, allowing for long-distance communication.
  • Line of Sight Propagation: Here, waves travel directly from the transmitter to the receiver. This mode is commonly used for high frequency signals like television and FM radio broadcasts.

Generation of Electromagnetic Waves

Electromagnetic waves are generated by oscillating electric charges. This involves an interplay between electric and magnetic fields. An oscillating charge creates an electric field, which in turn generates a magnetic field. The continuous interplay between these two fields leads to the propagation of the wave.

In summary, electromagnetic wave propagation is a complex process, crucial to many aspects of our lives. Understanding its characteristics and mechanisms can deepen our knowledge of the physical world and enhance our ability to harness these waves for various applications.

Related Posts:

Inductor

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em waves travel through space

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Introduction to the Electromagnetic Spectrum

What is Electromagnetic energy?

Electromagnetic energy travels in waves and spans a broad spectrum from very long radio waves to very short gamma rays. The human eye can only detect only a small portion of this spectrum called visible light. A radio detects a different portion of the spectrum, and an x-ray machine uses yet another portion. NASA's scientific instruments use the full range of the electromagnetic spectrum to study the Earth, the solar system, and the universe beyond.

When you tune your radio, watch TV, send a text message, or pop popcorn in a microwave oven, you are using electromagnetic energy. You depend on this energy every hour of every day. Without it, the world you know could not exist.

A diagram of the electromagnetic spectrum showing the 7 regions of the spectrum (from longest to shortest wavelength):  radio waves, microwave, infrared, visible, ultraviolet, x-ray, and gamma rays. Examples of radio waves include fm & am radio frequencies. Mircrowaves are used in microwave ovens and their lengths are about the size of a baseball. Infrared waves are much shorter – about the size of a thickness of paper. The heat energy that radiates off human and other animals is in the infrared region.

Our Protective Atmosphere

Our Sun is a source of energy across the full spectrum, and its electromagnetic radiation bombards our atmosphere constantly. However, the Earth's atmosphere protects us from exposure to a range of higher energy waves that can be harmful to life. Gamma rays, x-rays, and some ultraviolet waves are "ionizing," meaning these waves have such a high energy that they can knock electrons out of atoms. Exposure to these high-energy waves can alter atoms and molecules and cause damage to cells in organic matter. These changes to cells can sometimes be helpful, as when radiation is used to kill cancer cells, and other times not, as when we get sunburned.

Atmospheric Windows

Image of NASA Spacecraft RHESSI

Electromagnetic radiation is reflected or absorbed mainly by several gases in the Earth's atmosphere, among the most important being water vapor, carbon dioxide, and ozone. Some radiation, such as visible light, largely passes (is transmitted) through the atmosphere. These regions of the spectrum with wavelengths that can pass through the atmosphere are referred to as "atmospheric windows." Some microwaves can even pass through clouds, which make them the best wavelength for transmitting satellite communication signals.

While our atmosphere is essential to protecting life on Earth and keeping the planet habitable, it is not very helpful when it comes to studying sources of high-energy radiation in space. Instruments have to be positioned above Earth's energy-absorbing atmosphere to "see" higher energy and even some lower energy light sources such as quasars.

Next: Anatomy of an Electromagnetic Wave

National Aeronautics and Space Administration, Science Mission Directorate. (2010). Introduction to the Electromagnetic Spectrum. Retrieved [insert date - e.g. August 10, 2016] , from NASA Science website: http://science.nasa.gov/ems/01_intro

Science Mission Directorate. "Introduction to the Electromagnetic Spectrum" NASA Science . 2010. National Aeronautics and Space Administration. [insert date - e.g. 10 Aug. 2016] http://science.nasa.gov/ems/01_intro

Discover More Topics From NASA

James Webb Space Telescope

The image is divided horizontally by an undulating line between a cloudscape forming a nebula along the bottom portion and a comparatively clear upper portion. Speckled across both portions is a starfield, showing innumerable stars of many sizes. The smallest of these are small, distant, and faint points of light. The largest of these appear larger, closer, brighter, and more fully resolved with 8-point diffraction spikes. The upper portion of the image is blueish, and has wispy translucent cloud-like streaks rising from the nebula below. The orangish cloudy formation in the bottom half varies in density and ranges from translucent to opaque. The stars vary in color, the majority of which have a blue or orange hue. The cloud-like structure of the nebula contains ridges, peaks, and valleys – an appearance very similar to a mountain range. Three long diffraction spikes from the top right edge of the image suggest the presence of a large star just out of view.

Perseverance Rover

em waves travel through space

Parker Solar Probe

em waves travel through space

Expert Voices

Why is the speed of light the way it is?

It's just plain weird.

Einstein's theory of special relativity tells us the speed of light is 186,000 miles per second (300 million meters per second).

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio , and author of " How to Die in Space ." He contributed this article to Space.com's Expert Voices: Op-Ed & Insights . 

We all know and love the speed of light — 299,792,458 meters per second — but why does it have the value that it does? Why isn't it some other number? And why do we care so much about some random speed of electromagnetic waves? Why did it become such a cornerstone of physics? 

Well, it's because the speed of light is just plain weird.

Related: Constant speed of light: Einstein's special relativity survives a high-energy test

Putting light to the test

The first person to realize that light does indeed have a speed at all was an astronomer by the name of Ole Romer. In the late 1600s, he was obsessed with some strange motions of the moon Io around Jupiter. Every once in a while, the great planet would block our view of its little moon, causing an eclipse, but the timing between eclipses seemed to change over the course of the year. Either something funky was happening with the orbit of Io — which seemed suspicious — or something else was afoot.

After a couple years of observations, Romer made the connection. When we see Io get eclipsed, we're in a certain position in our own orbit around the sun. But by the next time we see another eclipse, a few days later, we're in a slightly different position, maybe closer or farther away from Jupiter than the last time. If we are farther away than the last time we saw an eclipse, then that means we have to wait a little bit of extra time to see the next one because it takes that much longer for the light to reach us, and the reverse is true if we happen to be a little bit closer to Jupiter.

The only way to explain the variations in the timing of eclipses of Io is if light has a finite speed.

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Making it mean something

Continued measurements over the course of the next few centuries solidified the measurement of the speed of light, but it wasn't until the mid-1800s when things really started to come together. That's when the physicist James Clerk Maxwell accidentally invented light.

Maxwell had been playing around with the then-poorly-understood phenomena of electricity and magnetism when he discovered a single unified picture that could explain all the disparate observations. Laying the groundwork for what we now understand to be the electromagnetic force , in those equations he discovered that changing electric fields can create magnetic fields, and vice versa. This allows waves of electricity to create waves of magnetism, which go on to make waves of electricity and back and forth and back and forth, leapfrogging over each other, capable of traveling through space.

And when he went to calculate the speed of these so-called electromagnetic waves, Maxwell got the same number that scientists had been measuring as the speed of light for centuries. Ergo, light is made of electromagnetic waves and it travels at that speed, because that is exactly how quickly waves of electricity and magnetism travel through space.

And this was all well and good until Einstein came along a few decades later and realized that the speed of light had nothing to do with light at all. With his special theory of relativity , Einstein realized the true connection between time and space, a unified fabric known as space-time. But as we all know, space is very different than time. A meter or a foot is very different than a second or a year. They appear to be two completely different things.

So how could they possibly be on the same footing?

There needed to be some sort of glue, some connection that allowed us to translate between movement in space and movement in time. In other words, we need to know how much one meter of space, for example, is worth in time. What's the exchange rate? Einstein found that there was a single constant, a certain speed, that could tell us how much space was equivalent to how much time, and vice versa.

Einstein's theories didn't say what that number was, but then he applied special relativity to the old equations of Maxwell and found that this conversion rate is exactly the speed of light.

Of course, this conversion rate, this fundamental constant that unifies space and time, doesn't know what an electromagnetic wave is, and it doesn't even really care. It's just some number, but it turns out that Maxwell had already calculated this number and discovered it without even knowing it. That's because all massless particles are able to travel at this speed, and since light is massless, it can travel at that speed. And so, the speed of light became an important cornerstone of modern physics.

But still, why that number, with that value, and not some other random number? Why did nature pick that one and no other? What's going on?

Related: The genius of Albert Einstein: his life, theories and impact on science

Making it meaningless

Well, the number doesn't really matter. It has units after all: meters per second. And in physics any number that has units attached to it can have any old value it wants, because it means you have to define what the units are. For example, in order to express the speed of light in meters per second, first you need to decide what the heck a meter is and what the heck a second is. And so the definition of the speed of light is tied up with the definitions of length and time.

In physics, we're more concerned with constants that have no units or dimensions — in other words, constants that appear in our physical theories that are just plain numbers. These appear much more fundamental, because they don't depend on any other definition. Another way of saying it is that, if we were to meet some alien civilization , we would have no way of understanding their measurement of the speed of light, but when it comes to dimensionless constants, we can all agree. They're just numbers.

One such number is known as the fine structure constant, which is a combination of the speed of light, Planck's constant , and something known as the permittivity of free space. Its value is approximately 0.007. 0.007 what? Just 0.007. Like I said, it's just a number.

So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is.

So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know.

Learn more by listening to the episode "Why is the speed of light the way it is?" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to Robert H, Michael E., @DesRon94, Evan W., Harry A., @twdixon, Hein P., Colin E., and Lothian53 for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

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  • voidpotentialenergy This is just my opinion but i think L speed is it's speed because the particle part of it is the fastest it can interact with the quanta distance in quantum fluctuation. Light is particle and wave so the wave happens in the void between quanta. Gravity probably travels in that void and why gravity seems instant. Reply
  • rod The space.com article wraps up the discussion with, "So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is. So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know." It seems that the *universe* made this decision, *But our universe has chosen the fine structure constant to be...* I did not know that the universe was capable of making decisions concerning constants used in physics. E=mc^2 is a serious constant. Look at nuclear weapons development, explosive yields, and stellar evolution burn rates for p-p chain and CNO fusion rates. The report indicates why alpha (fine structure constant) is what it is and c is what it is, *We don't know*. Reply
Admin said: We all know and love the speed of light, but why does it have the value that it does? Why isn't it some other number? And why did it become such a cornerstone of physics? Why is the speed of light the way it is? : Read more
rod said: The space.com article wraps up the discussion with, "So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is. So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know." It seems that the *universe* made this decision, *But our universe has chosen the fine structure constant to be...* I did not know that the universe was capable of making decisions concerning constants used in physics. E=mc^2 is a serious constant. Look at nuclear weapons development, explosive yields, and stellar evolution burn rates for p-p chain and CNO fusion rates. The report indicates why alpha (fine structure constant) is what it is and c is what it is, *We don't know*.
  • rod FYI. When someone says *the universe has chosen*, I am reminded of these five lessons from a 1982 Fed. court trial. The essential characteristics of science are: It is guided by natural law; It has to be explanatory by reference to natural law; It is testable against the empirical world; Its conclusions are tentative, i.e., are not necessarily the final word; and It is falsifiable. Five important points about science. Reply
  • Gary If the universe is expanding , how can the speed of light be constant ( miles per second , if each mile is getting longer ) ? Can light's velocity be constant while the universe expands ? So, with the expansion of the universe , doesn't the speed of light need to increase in order to stay at a constant velocity in miles per second ? Or, do the miles in the universe remain the same length as the universe 'adds' miles to its diameter ? Are the miles lengthening or are they simply being added / compounded ? Reply
  • Gary Lets say we're in outer space and we shoot a laser through a block of glass. What causes the speed of the laser light to return to the speed it held prior to entering the block of glass ? Is there some medium in the vacuum of space that governs the speed of light ? Do the atoms in the glass push it back up to its original speed. If so, why don't those same atoms constantly push the light while it travels through the block of glass ? Reply
Gary said: Lets say we're in outer space and we shoot a laser through a block of glass. What causes the speed of the laser light to return to the speed it held prior to entering the block of glass ? Is there some medium in the vacuum of space that governs the speed of light ? Do the atoms in the glass push it back up to its original speed. If so, why don't those same atoms constantly push the light while it travels through the block of glass ?
Gary said: If the universe is expanding , how can the speed of light be constant ( miles per second , if each mile is getting longer ) ? Can light's velocity be constant while the universe expands ? So, with the expansion of the universe , doesn't the speed of light need to increase in order to stay at a constant velocity in miles per second ? Or, do the miles in the universe remain the same length as the universe 'adds' miles to its diameter ? Are the miles lengthening or are they simply being added / compounded ?
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em waves travel through space

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Electromagnetic waves are a form of radiation that travel though the universe. They are formed when an electric field (Fig. 1 red arrows) couples with a magnetic field (Fig.1 blue arrows).

Both electricity and magnetism can be static (respectively, what holds a balloon to the wall or a refrigerator magnet to metal), but when they change or move together, they make waves. Magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave.

Unlike sound waves, which must travel through matter by bumping molecules into each other like dominoes (and thus can not travel through a vacuum like space), electromagnetic waves do not need molecules to travel. They can travel through air, solid objects, and even space, making them very useful for a lot of technologies.

When you listen to the radio, connect to a wireless network, or cook dinner in a microwave oven, you are using electromagnetic waves. Radio waves and microwaves are two types of electromagnetic waves. They only differ from each other in wavelength – the distance between one wave crest to the next.

While most of this energy is invisible to us, we can see the range of wavelengths that we call light. This visible part of the electromagnetic spectrum consists of the colors that we see in a rainbow – red, orange, yellow, green, blue, indigo, and violet. Each of these colors also corresponds to a different measurable wavelength of light.

Waves in the electromagnetic spectrum vary in size from very long radio waves that are the length of buildings to very short gamma-rays that are smaller than the nucleus of an atom.

Their size is related to their energy. The smaller the wavelength, the higher the energy. For example, a brick wall blocks the relatively larger and lower-energy wavelengths of visible light but not the smaller, more energetic x-rays. A denser material such as lead, however, can block x-rays.

While it’s commonly said that waves are "blocked" by certain materials, the correct understanding is that wavelengths of energy are absorbed by the material. This understanding is critical to interpreting data from weather satellites because the atmosphere also absorbs some wavelengths while allowing others to pass through.

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Emily Waltz is the power and energy editor at IEEE Spectrum.

A photo of a smiling woman with an image of the sun behind her.

Yilu Liu is one of the researchers at Oak Ridge National Laboratory studying how buildings (and their electrical circuits) can be protected from electromagnetic pulses.

This year, the sun will reach solar maximum , a period of peak magnetic activity that occurs approximately once every 11 years. That means more sunspots and more frequent intense solar storms. Here on Earth, these result in beautiful auroral activity, but also geomagnetic storms and the threat of electromagnetic pulses (EMPs), which can bring widespread damage to electronic equipment and communications systems.

Yilu Liu is a Governor’s Chair/Professor at the University of Tennessee, in Knoxville, and Oak Ridge National Laboratory.

And the sun isn’t the only source of EMPs. Human-made EMP generators mounted on trucks or aircraft can be used as tactical weapons to knock out drones , satellites , and infrastructure. More seriously, a nuclear weapon detonated at a high altitude could, among its more catastrophic effects, generate a wide-ranging EMP blast. IEEE Spectrum spoke with Yilu Liu , who has been researching EMPs at Oak Ridge National Laboratory, in Tennessee, about the potential effects of the phenomenon on power grids and other electronics.

What are the differences between various kinds of EMPs?

Yilu Liu: A nuclear explosion at an altitude higher than 30 kilometers would generate an EMP with a much broader spectrum than one from a ground-level weapon or a geomagnetic storm, and it would arrive in three phases. First comes E1, a powerful pulse that brings very fast high-frequency waves. The second phase, E2, produces current similar to that of a lightning strike. The third phase, E3, brings a slow, varying waveform, kind of like direct current [DC], that can last several minutes. A ground-level electromagnetic weapon would probably be designed for emitting high-frequency waves similar to those produced by an E1. Solar magnetic disturbances produce a slow, varying waveform similar to that of E3.

How do EMPs damage power grids and electronic equipment?

Liu: Phase E1 induces current in conductors that travels to sensitive electronic circuits, destroying them or causing malfunctions. We don’t worry about E2 much because it’s like lightning, and grids are protected against that. Phase E3 and solar magnetic EMPs inject a foreign, DC-like current into transmission lines, which saturates transformers, causing a lot of high-frequency currents that have led to blackouts.

How do you study the effects of an EMP without generating one?

Liu: We measured the propagation into a building of low-level electromagnetic waves from broadcast radio. We wanted to know if physical structures, like buildings, could act as a filter, so we took measurements of radio signals both inside and outside a hydropower station and other buildings to figure out how much gets inside. Our computer models then amplified the measurements to simulate how an EMP would affect equipment.

What did you learn about protecting buildings from damage by EMPs?

Liu: When constructing buildings, definitely use rebar in your concrete. It’s very effective as a shield against electromagnetic waves. Large windows are entry points, so don’t put unshielded control circuits near them. And if there are cables coming into the building carrying power or communication, make sure they are well-shielded; otherwise, they will act like antennas.

Have solar EMPs caused damage in the past?

Liu: The most destructive recent occurrence was in Quebec in 1989 , which resulted in a blackout. Once a transformer is saturated, the current flowing into the grid is no longer just 60 hertz but multiples of 60 Hz, and it trips the capacitors, and then the voltage collapses and the grid experiences an outage. The industry is better prepared now. But you never know if the next solar storm will surpass those of the past.

This article appears in the June 2024 issues as “5 Questions for Yilu Liu.”

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Emily Waltz  is a features editor at Spectrum covering power and energy. Prior to joining the staff in January 2024, Emily spent 18 years as a freelance journalist covering biotechnology, primarily for the  Nature  research journals and Spectrum . Her work has also appeared in Scientific American , Discover , Outside , and the New York Times . Emily has a master's degree from Columbia University Graduate School of Journalism and an undergraduate degree from Vanderbilt University. With every word she writes, Emily strives to say something true and useful. She posts on Twitter/X  @EmWaltz  and her portfolio can be found on her  website .

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IMAGES

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    em waves travel through space

  2. Exploring How Electromagnetic Waves Travel Through Space

    em waves travel through space

  3. Electromagnetic Waves

    em waves travel through space

  4. Success in Visualizing the Propagation Path of Electromagnetic Waves

    em waves travel through space

  5. Electromagnetic Waves

    em waves travel through space

  6. How do electromagnetic waves wave?

    em waves travel through space

VIDEO

  1. Propagation of EM waves in Dielectric Media // Unbounded Media // The Physics Family

  2. How are Gravitational waves detected??

  3. 10.2. EM Waves in General (Example)

  4. InnoSpaceTool 3: Electromagnetic Waves

  5. What speed do EM waves travel?

  6. Propagation of electromagnetic waves

COMMENTS

  1. 23.2: Electromagnetic Waves and their Properties

    Maxwell's correction shows that self-sustaining electromagnetic waves (light) can travel through empty space even in the absence of moving charges or currents, with the electric field component and magnetic field component each continually changing and each perpetuating the other. ... Electromagnetic Wave: Electromagnetic waves are a self ...

  2. Anatomy of an Electromagnetic Wave

    This means that electromagnetic waves can travel not only through air and solid materials, but also through the vacuum of space. In the 1860's and 1870's, a Scottish scientist named James Clerk Maxwell developed a scientific theory to explain electromagnetic waves. He noticed that electrical fields and magnetic fields can couple together to ...

  3. Light: Electromagnetic waves, the electromagnetic spectrum and photons

    Electromagnetic radiation is one of the many ways that energy travels through space. The heat from a burning fire, the light from the sun, the X-rays used by your doctor, as well as the energy used to cook food in a microwave are all forms of electromagnetic radiation. While these forms of energy might seem quite different from one another ...

  4. Electromagnetic radiation

    In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy.. Classically, electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields.In a vacuum, electromagnetic waves travel at the speed of light ...

  5. Chapter 6: Electromagnetics

    Electromagnetic radiation (radio waves, light, etc.) consists of interacting, self-sustaining electric and magnetic fields that propagate through empty space at 299,792 km per second (the speed of light, c), and slightly slower through air and other media.Thermonuclear reactions in the cores of stars (including the Sun) provide the energy that eventually leaves stars, primarily in the form of ...

  6. Ch. 24 Introduction to Electromagnetic Waves

    Conversely, radio waves are electromagnetic waves, like visible light, infrared, ultraviolet, X-rays, and gamma rays. EM waves don't need a medium in which to propagate; they can travel through a vacuum, such as outer space. A radio works because sound waves played by the D.J. at the radio station are converted into electromagnetic waves ...

  7. Astronomy & Astrophysics 101: Electromagnetic Spectrum

    From long to short wavelength, the EM spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays. Energy travels through space as electromagnetic (EM) waves, which are made up of oscillating electric and magnetic fields. EM waves do not require a material (such as air or water) to move through, hence ...

  8. Electromagnetic Radiation

    Electromagnetic radiation can travel through empty space. Most other types of waves must travel through some sort of substance. For example, sound waves need either a gas, solid, or liquid to pass through in order to be heard; The speed of light is always a constant (3 x 10^8 m/s)

  9. Electromagnetic waves

    The speed of any electromagnetic waves in free space is the speed of light c = 3*10 8 m/s. Electromagnetic waves can have any wavelength λ or frequency f as long as λf = c. When electromagnetic waves travel through a medium, the speed of the waves in the medium is v = c/n(λ free), where n(λ free) is the

  10. Electromagnetic waves and the electromagnetic spectrum

    And the speed at which these waves travel is the speed of light, c, and by c I mean three times 10 to the eight meters per second, because light is just and Electromagnetic wave, light is a special example, one particular example of Electromagnetic waves, but it is only one example, these waves can have any wavelength.

  11. The Electromagnetic Spectrum

    All light, or electromagnetic radiation, travels through space at 186,000 miles (300,000 kilometers) per second — the speed of light. That's about as far as a car will go over its lifetime, traveled by light in a single second! How We Measure Light. Light travels in waves, much like the waves you find in the ocean.

  12. The Physics Classroom Website

    Propagation of an Electromagnetic Wave. Electromagnetic waves are waves which can travel through the vacuum of outer space. Mechanical waves, unlike electromagnetic waves, require the presence of a material medium in order to transport their energy from one location to another. Sound waves are examples of mechanical waves while light waves are ...

  13. How does electromagnetic wave propagation work?

    Electromagnetic (EM) wave propagation is an essential concept in physics that describes how electromagnetic waves travel through space. These waves, which include light, radio, and X-rays, are fundamental to many aspects of our daily life. Characteristics of Electromagnetic Waves. Transverse Nature: EM waves are transverse in nature, meaning ...

  14. Introduction to the Electromagnetic Spectrum

    What is Electromagnetic energy? Electromagnetic energy travels in waves and spans a broad spectrum from very long radio waves to very short gamma rays. The human eye can only detect only a small portion of this spectrum called visible light. A radio detects a different portion of the spectrum, and an x-ray machine uses yet another portion.

  15. Why is the speed of light the way it is?

    Ergo, light is made of electromagnetic waves and it travels at that speed, because that is exactly how quickly waves of electricity and magnetism travel through space. And this was all well and ...

  16. Electromagnetic waves

    They can travel through air, solid objects, and even space, making them very useful for a lot of technologies. When you listen to the radio, connect to a wireless network, or cook dinner in a microwave oven, you are using electromagnetic waves. Radio waves and microwaves are two types of electromagnetic waves. They only differ from each other ...

  17. visible light

    0. Since, electro magnetic waves have electric and magnetic vector. Due to this EM waves show electric and magnetic field. An electric and magnetic field have no need a medium to show thier effect. Hence in the presence of electric and magnetic field vector which vibrate perpendeculer to each other and get pertervation EM waves travels in vacuum.

  18. EM waves and the electromagnetic spectrum

    can travel through a vacuum close vacuum A volume that contains no matter. such as in space travel at the same speed through a vacuum or the air Electromagnetic waves travel at 300,000,000 metres ...

  19. Electromagnetic Pulses: How to EMP-Proof a Building

    Yilu Liu: A nuclear explosion at an altitude higher than 30 kilometers would generate an EMP with a much broader spectrum than one from a ground-level weapon or a geomagnetic storm, and it would ...