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Electromagnetic radiation is a combination of oscillating electric and magnetic fields propagating through space and carrying energy from one place to another. Light is a form of electromagnetic radiation. The theoretical study of electromagnetic radiation is called electrodynamics, a subfield of electromagnetism.
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 velocity 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 = h f, where E is the energy of the photon, h is Planck's constant and f is the frequency of the wave.
Generally, electromagnetic radiation is classified by wavelength into radio, microwave, visible light, X-rays and gamma rays. The details of this classification are contained in the article on the electromagnetic spectrum.
The effect of radiation depends on the amount of energy per quantum it carries. High energies correspond to high frequencies and short wavelengths, and vice versa. One rule is always obeyed, regardless of the circumstances. Radiation in 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).
Much information about the physical properties of an object can be obtained from its electromagnetic spectrum; this can be either the spectrum of light emitted from, or transmitted through the object. This involves spectroscopy and is widely used in astrophysics. For example; many hydrogen atoms emit radio waves which have a wavelength of 21.12 cm.
When a electromagnetic radiation passes through a conductor it induces an electric current flow in the conductor. This effect is used in antennas. Electromagnetic radiation may also cause certain molecules to oscillate and thus to heat up; this is exploited in microwave ovens.