A thin film of alcohol (n = 1.36) lies on a flat glass plate (n = 1.51). When monochromatic light, whose wavelength λ can be changed, is incident normally, the reflected light is minimum for wavelength λ = 512 nm and a maximum for wavelength = 640 nm. What is the minimum thickness of the film?

The thickness of the film, t = 470.59 nm

Explanation:

For constructive interference, the net phase change,

Then the thickness,  

m₁ = 1,2,3........

..............(i)

For destructive interference, the net phase change is  

m₂ = 1,2,3........

thickness, ..............(ii)

Equating (i) and (ii)

.............(iii)

From (iii), When m₁ =2, m₂ = 2

The thickness , then becomes

t = 470.59 nm

thermal energy is the answer