Question

A very thin oil film (n=1.25) floats on water (n=1.33).

What is the thinnest film that produces a strong reflection for green light with a wavelength of 500 nm?

Answer 1

Data:
Wavelength of light, λ = 500 nm
                                 = 500 x 10^-9 m
Refractive index of oil, n1 = 1.25
Refractive index of water, n2 = 1.33
Thickness of the film, d = ?

Solution:

         For strong reflection, m = 1

         Wavelength, λ= 2 n1 d / m

         Thickness, d = λ m / 2 n1
                           = 500 x 10 ^-9 * 1 / ( 2 * 1.25 )
                          = 200 x 10 ^-9 m
                           = 200 nm

Ans:

Thickness of the film, d = 200 nm