Light is visible radiation as perceived by the human eye. As such it constitutes only a small segment of the total spectrum of electromagnetic radiation, which comprises not only visible light but also gamma radiation, x-rays, infrared and UV radiation, and radio waves, etc. The spectral range of light is 380 -780 nm. Within this range of wavelengths the colors of the spectrum can be distinguished, i.e. red, orange, yellow, green, blue, indigo and violet. Radiation outside of this range cannot be seen by the human eye.

Although radiation at many wavelengths cannot be perceived by the human eye, the human organism does react to some of them. Infrared radiation, for example, is perceived as heat, while UV-A radiation tans the skin.