Light is part of the electromagnetic spectrum.
This ranges from the short waves of cosmic rays to long wave radiation. Light itself forms a very small part of this spectrum, from 400 to 700 nm (nm= nanometers = 1 millionth of a mm = 10^-7cms ). Light is perceived as colours from violet to red. However it is important to understand that colour is a continuous blending without the apparently defined colours shown in the diagram.
The colours we normally give to the spectrum are only a convention to describe our perception of wavelengths. A certain mixture of all of these wavelengths gives white light. Light has certain properties, which are important in the understanding of photography.
1. Light radiates from a point source in straight lines.
2. It can pass though a vacuum or transparent substances such as glass or water.
3. It travels at very high speed (299 793 km s^-1), this is instantaneous as far as photography is concerned.
4. Light can be bent or refracted by transparent substances such as glass or reflected by mirror like surfaces.
5. Light falls off according to the inverse square law.
6. Light may react differently when it meets a surface; it may be absorbed, reflected, transmitted, reflected, or dispersed. This is what gives an object qualities such as colour or transparency.