1. Form of radiant energy that acts upon the retina of the eye, optic nerve, etc., making sight possible. This energy is transmitted at a velocity of about 186,000 miles per second by wavelike or vibrational motion.
2. A form of radiant energy similar to this, but not acting on the normal retina, such as ultraviolet and infrared radiation.
Interplay between light rays and the atmosphere cause us to see the sky as blue, and can result in such phenomena as glows, halos, arcs, flashes, and streamers.
Source: NASA (http://earthobservatory.nasa.gov/Glossary)