<p>Light is defined as the visible radiation perceptible by the human eye. It is a small part of the electromagnetic radiation spectrum that includes visible light plus gamma rays, X-rays, infrared radiation, UV radiation and radio waves. The spectrum of light ranges from 380 to 780 nanometers wavelength. Within this range, the spectrum is divided into the different colors from purple to blue, green, yellow to red. Outside this range the human eye can "see" no radiation. Some other areas of the electromagnetic spectrum, although invisible to the eye, but can be nevertheless perceived: infrared radiation warms, UV-A radiation tans.</p>