Find the minimum thickness of a film which will strongly reflect the light of wavelength 589 nm. The refractive index of the material of the film is 1.25.

Explanation

For strong reflection,the least optical path difference introduced by the film should be `lamda/2`. The optical path difference between the waves reflected from the two surfaces of the film is `2mud`. Thus, for strong reflection,
`2mud=lamda/2`
`or, d=lamda/(4lamda)=(589nm)/(4xx1.25)=118nm.