Question

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.
A. `118nm`
B. `120nm`
C. `118m`
D. `188nm`

Answer 1

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