Coherent and Incoherent Addition of Waves

By super position theorem, when two or more waves simultaneously pass through the same medium, each wave acts on every particle of the medium, as if the other waves are not present. The resultant displacement of any particle is the vector addition of the displacements due to the individual waves.

Two sources are said to be coherent if they emit light waves of the same wave length and start with same phase or have a constant phase difference.

Two independent monochromatic sources, emit waves of same wave length. But the waves are not in phase. So they are not coherent. This is because, atoms cannot emit light waves in same phase and these sources are said to be incoherent sources.

A wavelength λ corresponds to a phase of 2π. A distance of δ corresponds to a phase of

φ =