Energy transmitted through space as waves or particles: Two examples of radiation can be heat radiation or the splitting up of light rays.

Electromagnetic radiation with wavelengths shorter than those of X-rays: Gamma radiation is given off by any type of origin, but particularly by a part of the electromagnetic spectrum with a wavelength no more than 1 × 10–11 metres

On either side of the band of visible radiation are electromagnetic radiations which are invisible, but which can nevertheless be perceived: Examples of invisible radiations are microwaves, x-rays, gamma rays, and ultraviolet rays.

People know about the stars because they can be seen and because of this, optical telescopes are the traditional instruments of astronomy; however, it must not be forgotten that light is only one form of electromagnetic radiation, and that it forms only a narrow band in the electromagnetic spectrum.