Question

State the condition for constructive and destructive interference of light rays reflected from different media.

Answer 1

Light in air, reflecting off a medium will undergo a 180° shift. On the other hand, light in oil, which has a higher n than water does, will have no phase shift if it reflects off an oil-water interface.

For constructive interference, two reflected waves have to be shifted by an integer multiple of wavelengths.  For constructive interference, path difference between two waves is m*lamda. In constructive interference, two waves of light add to each other.

For destructive interference,  If the path difference between two light waves is (m+1/2)*lamda , then the interference between them will be destructive.  In destructive interference, two waves cancel the effects of each other
   Path difference = (m+1/2)*lamda