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

Options

A. 118 nm
B. 120 nm
C. 218 m
D. 225 mm

Answer

A

Explanation

For strong reflection, the least optical path difference introduced by the film should by `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`.
`d=lamda/(4mu)=(589)/(4xx1.25)=118nm`.