Electromagnetic radiation or EM radiation is a combination (cross product) of oscillating electric and magnetic fields perpendicular to each other, moving through space as a wave, effectively transporting energy and momentum. EM radiation is quantized as particles called photons. EM radiation with a wavelength between 400 nm and 700 nm is detected by the human eye and perceived as visible light. The physics of electromagnetic radiation is electrodynamics, a subfield of electromagnetism.
Electromagnetic waves were predicted by Maxwell's equations and subsequently discovered by Heinrich Hertz.
Any electric charge which accelerates, or any changing magnetic field, produces electromagnetic radiation. Electromagnetic information about the charge travels at the speed of light. Accurate treatment thus incorporates a concept known as retarded time (as opposed to advanced time, which is unphysical in light of causality), which adds to the expressions for the electrodynamic electric field and magnetic field. These extra terms are responsible for electromagnetic radiation. When any wire (or other conducting object such as an antenna) conducts alternating current, electromagnetic radiation is propagated at the same frequency as the electric current. Depending on the circumstances, it may behave as waves or as particles. As a wave, it is characterized by a velocity (the speed of light), wavelength, and frequency. When considered as particles, they are known as photons, and each has an energy related to the frequency of the wave given by Planck's relation E = hν, where E is the energy of the photon, h = 6.626 × 10-34 J·s is Planck's constant, and ν is the frequency of the wave.
Generally, EM radiation is classified by wavelength into electrical energy, radio, microwave, infrared, the visible region, (we perceive as light,) ultraviolet, X-rays and gamma rays. The details of this classification are contained in the article on the electromagnetic spectrum.
The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and longer wavelengths have lower frequencies. When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. One rule is always obeyed, regardless of the circumstances. EM radiation in a vacuum always travels at the speed of light, relative to the observer, regardless of the observer's velocity. (This observation led to Albert Einstein's development of the theory of special relativity).
Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400 nm to 700 nm. A common laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm. More in-depth information about the physical properties of objects, gases, or even stars can be obtained from this type of device. It is widely used in astrophysics. For example, many hydrogen atoms emit radio waves which have a wavelength of 21.12 cm.
If radiation having a frequency in the visible region of the EM spectrum shines on an object, say a bowl of fruit, this results in our visual perception identifying information from the scene. Our brain's visual system processes the multitude of reflected frequencies into different shades and hues, and through this not-entirely-explained "psychophysical phenomenon," most humans perceive a bowl of fruit.
In the vast majority of cases, however, the information carried by light is not directly apprehensible by human senses. Natural sources produce EM radiations across the spectrum; so, too, can human technology manipulate a broad range of wavelengths. Optical fiber transmits light which, although not suitable for direct viewing, can carry data. Those data can be translated into sound or even into an image. The coded form of such data is similar to that used with radio waves. Radio waves carry information by varying amplitude and by varying frequency within a frequency band.
When EM radiation impinges upon a conductor, it couples to the conductor, travels along it, and induces an electric current on the surface of that conductor. This effect (the skin effect) is used in antennas. EM radiation may also cause certain molecules to absorb energy and thus to heat up; this is exploited in microwave ovens.
The term "electromagnetic radiation" is also used as a synonym for electromagnetic waves in general. EM waves are called EM radiation even if the waves are not radiating or travelling in free space, e.g. if light travels through an optical fiber, or when electrical energy travels within a coaxial cable.