Interference of light: Wave emitted from two coherent sources having the same wavelength, equal or nearly equal amplitude when passes through a point in a medium, then superposition takes place.
During superposition, if the two waves reaching the point in the same phase then the intensity will be maximum that is bright. But if the two waves reaching the point in opposite phase then the point will have minimum intensity that is dark. This variation of intensity due to superposition is called interference of light.
The interference is said to be constructive or destructive when the intensity is respectively more or less than that given by one beam alone.