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.