Question

A parallel beam of light of wavelength 560 nm falls on a thin film of oil (refractive index = 1.4). What should be the minimum thickness of the film so that it strongly reflects the light?

Answer 1

Given:

Wavelength of  light used, λ=560×10⁻⁹m

Refractive index of the oil film, μ=1.4

Let the thickness of the film for strong reflection be t.

The condition for strong reflection is

2μt=(2n+1)λ/2

⇒t=(2n+1)λ/4μ  where n is an integer.

For minimum thickness, putting n = 0, we get:

t=λ/4μ     =560×10⁻⁹/ 4×1.4     =10⁻⁷m=100 nm

Therefore, the minimum thickness of the oil film so that it strongly reflects the light is 100 nm.

Answer 2

Explanation:

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