The electromagnetic spectrum consists of gamma rays, X-rays, ultraviolet radiation, visible light, infrared, and radio radiation. Light is one form of this electromagnetic radiation.

Electromagnetic radiation sometimes behaves like waves, but at other times, it behaves as if it were a particle—a little packet of energy, called a photon. The apparent brightness of a source of electromagnetic energy decreases with increasing distance from that source in proportion to the square of the distance—a relationship known as the inverse square law

ion — an atom that has become electrically charged by the addition or loss of one or more electrons. electromagnetic radiation — radiation consisting of waves propagated through regularly varying electric and magnetic fields and traveling at the speed of light. gamma rays — photons (of electromagnetic radiation) of energy with wavelengths no longer than 0.01 nanometer; the most energetic form of electromagnetic radiation. infrared — electromagnetic radiation of wavelength 10 3 -10 6 nanometers; longer than the longest (red) wavelengths that can be perceived by the eye, but shorter than radio wavelengths.