Electromagnetic wave equation

Overview

**electromagnetic wave equation**is a second-order partial differential equation

Partial differential equation

In mathematics, partial differential equations are a type of differential equation, i.e., a relation involving an unknown function of several independent variables and their partial derivatives with respect to those variables...

that describes the propagation of electromagnetic waves through a medium

Medium (optics)

An optical medium is material through which electromagnetic waves propagate. It is a form of transmission medium. The permittivity and permeability of the medium define how electromagnetic waves propagate in it...

or in a vacuum

Vacuum

In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

. The homogeneous

Homogeneous differential equation

The term homogeneous differential equation has several distinct meanings.One meaning is that a first-order ordinary differential equation is homogeneous if it has the formwhere F is a homogeneous function of degree zero; that is to say, that F = F.In a related, but distinct, usage, the term linear...

form of the equation, written in terms of either the electric field

Electric field

In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

**E**or the magnetic field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

**B**, takes the form:

where

is the speed of light

Speed of light

The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

in the medium, and ∇

^{2}is the Laplace operator

Vector Laplacian

In mathematics and physics, the vector Laplace operator, denoted by \scriptstyle \nabla^2, named after Pierre-Simon Laplace, is a differential operator defined over a vector field. The vector Laplacian is similar to the scalar Laplacian...

.

Unanswered Questions

Encyclopedia

The

that describes the propagation of electromagnetic waves through a medium

or in a vacuum

. The homogeneous

form of the equation, written in terms of either the electric field

where

is the speed of light

in the medium, and ∇

. In a vacuum,

. It should also be noted that in most older literature,

, Maxwell utilized the correction to Ampère's circuital law that he had made in part III of his 1861 paper On Physical Lines of Force. In

Maxwell's derivation of the electromagnetic wave equation has been replaced in modern physics by a much less cumbersome method involving combining the corrected version of Ampère's circuital law with Faraday's law of induction

.

To obtain the electromagnetic wave equation in a vacuum using the modern method, we begin with the modern 'Heaviside' form of Maxwell's equations. In a vacuum and charge free space, these equations are:

where ρ = 0 because there's no charge density in free space.

Taking the curl of the curl equations gives:

By using the vector identity

where is any vector function of space, it turns into the wave equations:

where

is the speed of light in free space.

can be written in contravariant form as

where the electromagnetic four-potential

is

with the Lorenz gauge condition

:

where

is the d'Alembertian operator. (The square box is not a typographical error; it is the correct symbol for this operator.)

and a new term that depends on the curvature appears.

where is the Ricci curvature tensor and the semicolon indicates covariant differentiation.

The generalization of the Lorenz gauge condition

in curved spacetime is assumed:

of waves of the form

and

for virtually

(in radians per second), and is the wave vector

(in radians per meter).

Although the function

, it does not have to be sinusoidal, or even periodic. In practice,

, a real wave must consist of the superposition of an infinite set of sinusoidal frequencies.

In addition, for a valid solution, the wave vector and the angular frequency are not independent; they must adhere to the dispersion relation

:

where

and λ is the wavelength

.

where

Then planar traveling wave solutions of the wave equations are

and

where is the position vector (in meters).

These solutions represent planar waves traveling in the direction of the normal vector . If we define the z direction as the direction of and the x direction as the direction of , then by Faraday's Law the magnetic field lies in the y direction and is related to the electric field by the relation . Because the divergence of the electric and magnetic fields are zero, there are no fields in the direction of propagation.

This solution is the linearly polarized solution of the wave equations. There are also circularly polarized solutions in which the fields rotate about the normal vector.

. This is the basis for the Fourier transform

method for the solution of differential equations.

The sinusoidal solution to the electromagnetic wave equation takes the form

and

where is time (in seconds), is the angular frequency

(in radians per second), is the wave vector

(in radians per meter), and is the phase angle

(in radians).

The wave vector is related to the angular frequency by

where

and λ is the wavelength

.

The electromagnetic spectrum

is a plot of the field magnitudes (or energies) as a function of wavelength.

for

with

A generic electromagnetic field with frequency ω can be written as a sum of solutions to these two equations. The three-dimensional solutions of the Helmholtz Equation can be expressed as expansions in spherical harmonics

with coefficients proportional to the spherical Bessel functions. However, applying this expansion to each vector component of

The multipole expansion circumvents this difficulty by expanding not

where and are the

where

normalized so that

The multipole expansion of the electromagnetic field finds application in a number of problems involving spherical symmetry, for example antennae radiation pattern

s, or nuclear gamma decay. In these applications, one is often interested in the power radiated in the far-field. In this regions, the

The angular distribution of the time-averaged radiated power is then given by

In spherical coordinates the solutions to the wave equation can be written as follows:

,

and

These can be rewritten in terms of the spherical bessel function

.

In cylindrical coordinates, the solutions to the wave equation are the ordinary bessel function

of integer order.

**electromagnetic wave equation**is a second-order partial differential equationPartial differential equation

In mathematics, partial differential equations are a type of differential equation, i.e., a relation involving an unknown function of several independent variables and their partial derivatives with respect to those variables...

that describes the propagation of electromagnetic waves through a medium

Medium (optics)

An optical medium is material through which electromagnetic waves propagate. It is a form of transmission medium. The permittivity and permeability of the medium define how electromagnetic waves propagate in it...

or in a vacuum

Vacuum

In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

. The homogeneous

Homogeneous differential equation

The term homogeneous differential equation has several distinct meanings.One meaning is that a first-order ordinary differential equation is homogeneous if it has the formwhere F is a homogeneous function of degree zero; that is to say, that F = F.In a related, but distinct, usage, the term linear...

form of the equation, written in terms of either the electric field

Electric field

In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

**E**or the magnetic fieldMagnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

**B**, takes the form:where

is the speed of light

Speed of light

The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

in the medium, and ∇

^{2}is the Laplace operatorVector Laplacian

In mathematics and physics, the vector Laplace operator, denoted by \scriptstyle \nabla^2, named after Pierre-Simon Laplace, is a differential operator defined over a vector field. The vector Laplacian is similar to the scalar Laplacian...

. In a vacuum,

*c*=*c*_{0}= 299,792,458 meters per second, which is the speed of light in free space. The electromagnetic wave equation derives from Maxwell's equationsMaxwell's equations

Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.Maxwell's equations...

. It should also be noted that in most older literature,

**B**is called the*magnetic flux density*or*magnetic induction*.## The origin of the electromagnetic wave equation

In his 1864 paper titled A Dynamical Theory of the Electromagnetic FieldA Dynamical Theory of the Electromagnetic Field

"A Dynamical Theory of the Electromagnetic Field" is the third of James Clerk Maxwell's papers regarding electromagnetism, published in 1865. It is the paper in which the original set of four Maxwell's equations first appeared...

, Maxwell utilized the correction to Ampère's circuital law that he had made in part III of his 1861 paper On Physical Lines of Force. In

*Part VI*of his 1864 paper titled*Electromagnetic Theory of Light*, Maxwell combined displacement current with some of the other equations of electromagnetism and he obtained a wave equation with a speed equal to the speed of light. He commented:*The agreement of the results seems to show that light and magnetism are affections of the same substance, and that light is an electromagnetic disturbance propagated through the field according to electromagnetic laws.*

Maxwell's derivation of the electromagnetic wave equation has been replaced in modern physics by a much less cumbersome method involving combining the corrected version of Ampère's circuital law with Faraday's law of induction

Faraday's law of induction

Faraday's law of induction dates from the 1830s, and is a basic law of electromagnetism relating to the operating principles of transformers, inductors, and many types of electrical motors and generators...

.

To obtain the electromagnetic wave equation in a vacuum using the modern method, we begin with the modern 'Heaviside' form of Maxwell's equations. In a vacuum and charge free space, these equations are:

where ρ = 0 because there's no charge density in free space.

Taking the curl of the curl equations gives:

By using the vector identity

where is any vector function of space, it turns into the wave equations:

where

is the speed of light in free space.

## Covariant form of the homogeneous wave equation

These relativistic equationsFormulation of Maxwell's equations in special relativity

The covariant formulation of classical electromagnetism refers to ways of writing the laws of classical electromagnetism in a form which is "manifestly covariant" , in the formalism of special relativity...

can be written in contravariant form as

where the electromagnetic four-potential

Electromagnetic four-potential

The electromagnetic four-potential is a potential from which the electromagnetic field can be derived. It combines both the electric scalar potential and the magnetic vector potential into a single space-time four-vector. In a given reference frame, the first component is the scalar potential and...

is

with the Lorenz gauge condition

Lorenz gauge condition

In electromagnetism, the Lorenz gauge or Lorenz gauge condition is a partial gauge fixing of the electromagnetic vector potential. The condition is that \partial_\mu A^\mu=0...

:

where

is the d'Alembertian operator. (The square box is not a typographical error; it is the correct symbol for this operator.)

## Homogeneous wave equation in curved spacetime

The electromagnetic wave equation is modified in two ways, the derivative is replaced with the covariant derivativeCovariant derivative

In mathematics, the covariant derivative is a way of specifying a derivative along tangent vectors of a manifold. Alternatively, the covariant derivative is a way of introducing and working with a connection on a manifold by means of a differential operator, to be contrasted with the approach given...

and a new term that depends on the curvature appears.

where is the Ricci curvature tensor and the semicolon indicates covariant differentiation.

The generalization of the Lorenz gauge condition

Lorenz gauge condition

In electromagnetism, the Lorenz gauge or Lorenz gauge condition is a partial gauge fixing of the electromagnetic vector potential. The condition is that \partial_\mu A^\mu=0...

in curved spacetime is assumed:

## Inhomogeneous electromagnetic wave equation

Localized time-varying charge and current densities can act as sources of electromagnetic waves in a vacuum. Maxwell's equations can be written in the form of a wave equation with sources. The addition of sources to the wave equations makes the partial differential equations inhomogeneous.## Solutions to the homogeneous electromagnetic wave equation

The general solution to the electromagnetic wave equation is a linear superpositionSuperposition principle

In physics and systems theory, the superposition principle , also known as superposition property, states that, for all linear systems, the net response at a given place and time caused by two or more stimuli is the sum of the responses which would have been caused by each stimulus individually...

of waves of the form

and

for virtually

*any*well-behaved function*g*of dimensionless argument φ, where is the angular frequencyAngular frequency

In physics, angular frequency ω is a scalar measure of rotation rate. Angular frequency is the magnitude of the vector quantity angular velocity...

(in radians per second), and is the wave vector

Wave vector

In physics, a wave vector is a vector which helps describe a wave. Like any vector, it has a magnitude and direction, both of which are important: Its magnitude is either the wavenumber or angular wavenumber of the wave , and its direction is ordinarily the direction of wave propagation In...

(in radians per meter).

Although the function

*g*can be and often is a monochromatic sine waveSine wave

The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...

, it does not have to be sinusoidal, or even periodic. In practice,

*g*cannot have infinite periodicity because any real electromagnetic wave must always have a finite extent in time and space. As a result, and based on the theory of Fourier decompositionFourier transform

In mathematics, Fourier analysis is a subject area which grew from the study of Fourier series. The subject began with the study of the way general functions may be represented by sums of simpler trigonometric functions...

, a real wave must consist of the superposition of an infinite set of sinusoidal frequencies.

In addition, for a valid solution, the wave vector and the angular frequency are not independent; they must adhere to the dispersion relation

Dispersion relation

In physics and electrical engineering, dispersion most often refers to frequency-dependent effects in wave propagation. Note, however, that there are several other uses of the word "dispersion" in the physical sciences....

:

where

*k*is the wavenumberWavenumber

In the physical sciences, the wavenumber is a property of a wave, its spatial frequency, that is proportional to the reciprocal of the wavelength. It is also the magnitude of the wave vector...

and λ is the wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

.

### Monochromatic, sinusoidal steady-state

The simplest set of solutions to the wave equation result from assuming sinusoidal waveforms of a single frequency in separable form:where

- is the imaginary unitImaginary unitIn mathematics, the imaginary unit allows the real number system ℝ to be extended to the complex number system ℂ, which in turn provides at least one root for every polynomial . The imaginary unit is denoted by , , or the Greek...

, -
**is the angular frequency**Angular frequencyIn physics, angular frequency ω is a scalar measure of rotation rate. Angular frequency is the magnitude of the vector quantity angular velocity...

in radians per second, **is the**frequencyFrequencyFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

in hertzHertzThe hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

, and- is Euler's formulaEuler's formulaEuler's formula, named after Leonhard Euler, is a mathematical formula in complex analysis that establishes the deep relationship between the trigonometric functions and the complex exponential function...

.

### Plane wave solutions

Consider a plane defined by a unit normal vectorThen planar traveling wave solutions of the wave equations are

and

where is the position vector (in meters).

These solutions represent planar waves traveling in the direction of the normal vector . If we define the z direction as the direction of and the x direction as the direction of , then by Faraday's Law the magnetic field lies in the y direction and is related to the electric field by the relation . Because the divergence of the electric and magnetic fields are zero, there are no fields in the direction of propagation.

This solution is the linearly polarized solution of the wave equations. There are also circularly polarized solutions in which the fields rotate about the normal vector.

### Spectral decomposition

Because of the linearity of Maxwell's equations in a vacuum, solutions can be decomposed into a superposition of sinusoidsSine

In mathematics, the sine function is a function of an angle. In a right triangle, sine gives the ratio of the length of the side opposite to an angle to the length of the hypotenuse.Sine is usually listed first amongst the trigonometric functions....

. This is the basis for the Fourier transform

Fourier transform

In mathematics, Fourier analysis is a subject area which grew from the study of Fourier series. The subject began with the study of the way general functions may be represented by sums of simpler trigonometric functions...

method for the solution of differential equations.

The sinusoidal solution to the electromagnetic wave equation takes the form

and

where is time (in seconds), is the angular frequency

Angular frequency

In physics, angular frequency ω is a scalar measure of rotation rate. Angular frequency is the magnitude of the vector quantity angular velocity...

(in radians per second), is the wave vector

Wave vector

In physics, a wave vector is a vector which helps describe a wave. Like any vector, it has a magnitude and direction, both of which are important: Its magnitude is either the wavenumber or angular wavenumber of the wave , and its direction is ordinarily the direction of wave propagation In...

(in radians per meter), and is the phase angle

Phase angle

In the context of vectors and phasors, the term phase angle refers to the angular component of the polar coordinate representation. The notation A\ang \!\ \theta, for a vector with magnitude A and phase angle θ, is called angle notation.In the context of periodic phenomena, such as a wave,...

(in radians).

The wave vector is related to the angular frequency by

where

*k*is the wavenumberWavenumber

In the physical sciences, the wavenumber is a property of a wave, its spatial frequency, that is proportional to the reciprocal of the wavelength. It is also the magnitude of the wave vector...

and λ is the wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

.

The electromagnetic spectrum

Electromagnetic spectrum

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

is a plot of the field magnitudes (or energies) as a function of wavelength.

### Multipole Expansion

Assuming monochromatic fields varying in time as , if one uses Maxwell's Equations to eliminate**B**, the electromagnetic wave equation reduces to the Helmholtz EquationHelmholtz equation

The Helmholtz equation, named for Hermann von Helmholtz, is the elliptic partial differential equation\nabla^2 A + k^2 A = 0where ∇2 is the Laplacian, k is the wavenumber, and A is the amplitude.-Motivation and uses:...

for

**E**:with

*k = ω/c*as given above. Alternatively, one can eliminate**E**in favor of**B**to obtain:A generic electromagnetic field with frequency ω can be written as a sum of solutions to these two equations. The three-dimensional solutions of the Helmholtz Equation can be expressed as expansions in spherical harmonics

Spherical harmonics

In mathematics, spherical harmonics are the angular portion of a set of solutions to Laplace's equation. Represented in a system of spherical coordinates, Laplace's spherical harmonics Y_\ell^m are a specific set of spherical harmonics that forms an orthogonal system, first introduced by Pierre...

with coefficients proportional to the spherical Bessel functions. However, applying this expansion to each vector component of

**E**or**B**will give solutions that are not generically divergence-free (**∇**·**E**=**∇**·**B**= 0), and therefore require additional restrictions on the coefficients.The multipole expansion circumvents this difficulty by expanding not

**E**or**B**, but**r · E**or**r · B**into spherical harmonics. These expansions still solve the original Helmholtz equations for**E**and**B**because for a divergence-free field**F**, ∇^{2}(**r · F**) =**r ·**(∇^{2}**F**). The resulting expressions for a generic electromagnetic field are:,where and are the

*electric multipole fields of order (l, m)*, and and are the corresponding*magnetic multipole fields*, and*a*and_{E}(l,m)*a*are the coefficients of the expansion. The multipole fields are given by,_{M}(l,m)where

*h*are the spherical Hankel functions,_{l}^{(1,2)}(x)*E*and_{l}^{(1,2)}*B*are determined by boundary conditions, and are vector spherical harmonics_{l}^{(1,2)}Vector spherical harmonics

In mathematics, vector spherical harmonics are an extension of the scalar spherical harmonics for the use with vector fields.-Definition:Several conventions have been used to define the VSH.We follow that of Barrera et al....

normalized so that

The multipole expansion of the electromagnetic field finds application in a number of problems involving spherical symmetry, for example antennae radiation pattern

Radiation pattern

In the field of antenna design the term radiation pattern most commonly refers to the directional dependence of the strength of the radio waves from the antenna or other source ....

s, or nuclear gamma decay. In these applications, one is often interested in the power radiated in the far-field. In this regions, the

**E**and**B**fields asymptote toThe angular distribution of the time-averaged radiated power is then given by

### Other solutions

Other spherically and cylindrically symmetric analytic solutions to the electromagnetic wave equations are also possible.In spherical coordinates the solutions to the wave equation can be written as follows:

,

and

These can be rewritten in terms of the spherical bessel function

Bessel function

In mathematics, Bessel functions, first defined by the mathematician Daniel Bernoulli and generalized by Friedrich Bessel, are canonical solutions y of Bessel's differential equation:...

.

In cylindrical coordinates, the solutions to the wave equation are the ordinary bessel function

Bessel function

In mathematics, Bessel functions, first defined by the mathematician Daniel Bernoulli and generalized by Friedrich Bessel, are canonical solutions y of Bessel's differential equation:...

of integer order.

### Theory and Experiment

- Maxwell's equationsMaxwell's equationsMaxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.Maxwell's equations...
- Wave equationWave equationThe wave equation is an important second-order linear partial differential equation for the description of waves – as they occur in physics – such as sound waves, light waves and water waves. It arises in fields like acoustics, electromagnetics, and fluid dynamics...
- Electromagnetic modeling
- Electromagnetic radiationElectromagnetic radiationElectromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
- Charge conservationCharge conservationIn physics, charge conservation is the principle that electric charge can neither be created nor destroyed. The net quantity of electric charge, the amount of positive charge minus the amount of negative charge in the universe, is always conserved...
- LightLightLight or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
- Electromagnetic spectrumElectromagnetic spectrumThe electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
- OpticsOpticsOptics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...

- Special relativitySpecial relativitySpecial relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
- General relativityGeneral relativityGeneral relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
- Photon dynamics in the double-slit experimentPhoton dynamics in the double-slit experimentThe dynamics of photons in the double-slit experiment describes the relationship between classical electromagnetic waves and photons, the quantum counterpart of classical electromagnetic waves, in the context of the double-slit experiment....
- Photon polarizationPhoton polarizationPhoton polarization is the quantum mechanical description of the classical polarized sinusoidal plane electromagnetic wave. Individual photons are completely polarized...
- Larmor power formulaLarmor formulaIn physics, in the area of electrodynamics, the Larmor formula is used to calculate the total power radiated by a nonrelativistic point charge as it accelerates. It was first derived by J. J...
- Theoretical and experimental justification for the Schrödinger equationTheoretical and experimental justification for the Schrödinger equationThe theoretical and experimental justification for the Schrödinger equation motivates the discovery of the Schrödinger equation, the equation that describes the dynamics of nonrelativistic particles...

### Applications

- RainbowRainbowA rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc...
- Cosmic microwave background radiationCosmic microwave background radiationIn cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....
- LaserLaserA laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
- Laser fusion
- PhotographyPhotographyPhotography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
- X-rayX-rayX-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
- X-ray crystallographyX-ray crystallographyX-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
- RADARRadarRadar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

- Radio wavesRadio wavesRadio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves,...
- Optical computing
- MicrowaveMicrowaveMicrowaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
- HolographyHolographyHolography is a technique that allows the light scattered from an object to be recorded and later reconstructed so that when an imaging system is placed in the reconstructed beam, an image of the object will be seen even when the object is no longer present...
- MicroscopeMicroscopeA microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
- TelescopeTelescopeA telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
- Gravitational lensGravitational lensA gravitational lens refers to a distribution of matter between a distant source and an observer, that is capable of bending the light from the source, as it travels towards the observer...
- Black body radiation

### Biographies

- André-Marie AmpèreAndré-Marie AmpèreAndré-Marie Ampère was a French physicist and mathematician who is generally regarded as one of the main discoverers of electromagnetism. The SI unit of measurement of electric current, the ampere, is named after him....
- Albert EinsteinAlbert EinsteinAlbert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...
- Michael FaradayMichael FaradayMichael Faraday, FRS was an English chemist and physicist who contributed to the fields of electromagnetism and electrochemistry....
- Heinrich Hertz
- Oliver HeavisideOliver HeavisideOliver Heaviside was a self-taught English electrical engineer, mathematician, and physicist who adapted complex numbers to the study of electrical circuits, invented mathematical techniques to the solution of differential equations , reformulated Maxwell's field equations in terms of electric and...
- James Clerk MaxwellJames Clerk MaxwellJames Clerk Maxwell of Glenlair was a Scottish physicist and mathematician. His most prominent achievement was formulating classical electromagnetic theory. This united all previously unrelated observations, experiments and equations of electricity, magnetism and optics into a consistent theory...

#### Journal articles

- Maxwell, James Clerk, "
*A Dynamical Theory of the Electromagnetic Field*", Philosophical Transactions of the Royal Society of London 155, 459-512 (1865). (This article accompanied a December 8, 1864 presentation by Maxwell to the Royal Society.)

#### Undergraduate-level textbooks

- Edward M. Purcell,
*Electricity and Magnetism*(McGraw-Hill, New York, 1985). ISBN 0-07-004908-4. - Hermann A. Haus and James R. Melcher,
*Electromagnetic Fields and Energy*(Prentice-Hall, 1989) ISBN 0-13-249020-X. - Banesh Hoffmann,
*Relativity and Its Roots*(Freeman, New York, 1983). ISBN 0-7167-1478-7. - David H. Staelin, Ann W. Morgenthaler, and Jin Au Kong,
*Electromagnetic Waves*(Prentice-Hall, 1994) ISBN 0-13-225871-4. - Charles F. Stevens,
*The Six Core Theories of Modern Physics*, (MIT Press, 1995) ISBN 0-262-69188-4. - Markus Zahn,
*Electromagnetic Field Theory: a problem solving approach*, (John Wiley & Sons, 1979) ISBN 0-471-02198-9

#### Graduate-level textbooks

- Landau, L. D.,
*The Classical Theory of Fields*(Course of Theoretical PhysicsCourse of Theoretical PhysicsThe Course of Theoretical Physics is a ten-volume series of books covering theoretical physics that was initiated by Lev Landau and written in collaboration with his student Evgeny Lifshitz starting in the late 1930s....

: Volume 2), (Butterworth-Heinemann: Oxford, 1987). ISBN 0-08-018176-7. - Charles W. Misner, Kip S. ThorneKip ThorneKip Stephen Thorne is an American theoretical physicist, known for his prolific contributions in gravitation physics and astrophysics and for having trained a generation of scientists...

, John Archibald WheelerJohn Archibald WheelerJohn Archibald Wheeler was an American theoretical physicist who was largely responsible for reviving interest in general relativity in the United States after World War II. Wheeler also worked with Niels Bohr in explaining the basic principles behind nuclear fission...

,*Gravitation*, (1970) W.H. Freeman, New York; ISBN 0-7167-0344-0.*(Provides a treatment of Maxwell's equations in terms of differential forms.)*

### Vector calculus

- P. C. Matthews
*Vector Calculus*, Springer 1998, ISBN 3-540-76180-2 - H. M. Schey,
*Div Grad Curl and all that: An informal text on vector calculus*, 4th edition (W. W. Norton & Company, 2005) ISBN 0-393-92516-1.