CBSE Class 12-science Answered

Interference of light occurs on account of the superposition of light waves from two coherent sources.

coherent sources are those sources of light which emit continuous light waves of same wavelength, same frequency and in same phase or having a constant phase difference.

Superposition of light waves from two incoherent sources does not result in interference.Even if two light waves of same wavelength and frequency superimpose they do not result in interference if the phase difference between the two waves is not a constant.

Hence for interference to occur the phase difference between the waves should remain a constant.Thus we can say that occurence of interference depends on the phase difference between the waves interfering.

Constructive interference occur when crest of one wave falls on crest of the other and trough of one wave falls on the trough of other.The resultant amplitude and hence intensity of light is maximum at these positions. The condition for constructive interference at a point is that the phase difference between the two waves reaching the point should be 0 or even integral multiple of 2π. (i.e .φ = 2 nπ, n=0,1,2.....) .

Destructive interference occur when crest of one wave falls on the trough of other and trough of one wave falls on the crest of other.The resultant amplitude and hence intensity of light is minimum at these points.The condition for destructive interference at a point is that the phase difference between the two waves reaching the point should be an odd integral multiple of π. (i.e .φ = (2 n - 1 ) π , n =1,2, .....) .