light Electromagnetic waves* in the wavelength ranging from 400 nm to 700 nm, to which human eye is sensitive, is called light. Newton believed that light is composed of corpuscles. The discovery of phenomena of interference, and diffraction by Young, Fresnel and others gave strong evidence that light has wave like character. Maxwell's investigations on electromagnetic theory showed that light is also a form of electromagnetic wave. An all pervading medium called ether was assumed in which light waves propagate. This ether hypothesis was rejected after the famous experiment by Michelson and Morley. This experiment showed that the velocity of light is constant, independent of the motion of the source or the observer. The result proved the non existence of ether.

Finally the discovery of photoelectric effect* and its subsequent explanation using the photon concept by Einstein led us to believe that light has dual characteristic of wave and particle. Even though it is essential to assign both wave and particle aspect to light (or to any other form of em radiation), in any given experimental situation only one aspect is applicable. This is known as principle of complementarity, which was stated by Niels Bohr as " the wave and particle aspects of electromagnetic radiation are complementary". That is in any experiment involving em radiation can be completely explained by either the wave or particle aspect.