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According to the wave theory of light, the light emitted from a source travels in the medium in the form of waves. With a single source of light, the distribution of light energy in the surrounding medium is uniform. When there are two sources, under certain conditions called coherent sources, the distribution of light energy is no longer uniform. There are certain regions where the intensity of light is maximum and there are also certain regions where the intensity of light is minimum. The energy due to the two sources is thus disturbed. This redistribution of light energy obtained by the superposition of light waves from two coherent sources of light is called interference of light. At points where the crest of one wave falls over the crest of the other wave or a trough of one wave falls over the trough of the other wave, the resultant amplitude of the resulting wave is maximum.


At such points the intensity of light is maximum and this is called constructive interference. Similarly at points where the crest of one wave falls over the trough of the other wave, the resultant amplitude of the resulting wave is minimum. At such point, the intensity of light is minimum  and this is called destructive interference.

Types of Interference 

Interference can be divided in the following two classes ;

a) Interference by division of wavefront: In this class, a wave-front is divided into two parts either by reflection or by refraction. These two parts obtained fro the same wavefront travel and interfere to produce interference pattern.

b) Interference by division of amplitude : In this class, the amplitude of beam of monochromatic light is divided into two parts either by partial reflection or refraction. The divided beams so obtained interfere and produce interference pattern.