electromagnetic radiation Energy in the form of transverse waves. It travels through free space at close to the speed of light, c.300,000km (186,000mi)/sec. In general, electromagnetic waves are set up by electrical and magnetic vibrations that occur universally in atoms. The wavelength of electromagnetic radiation varies inversely with the wave frequency. These waves make up the electromagnetic spectrum, which includes (in ascending order of frequency): radio waves (including microwaves), infrared waves, light, ultraviolet radiation, X-rays, and gamma radiation. They can undergo reflection, refraction, interference, diffraction, and polarization. In some cases, such as the photoelectric effect, electromagnetic radiation behaves in a way that can only be explained by assuming the radiation is composed of quanta of energy (photons). See also quantum theory

electromagnetic radiation (EMR) The range in radiation extending from wavelengths of less than 10⁻¹²m to more than 10³m. In order of increasing wavelength are included cosmic ray photons, gamma rays, X-rays, ultraviolet radiation, visible light (violet to red), infrared radiation, microwaves, radio waves, and electric currents.