Question

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

Answer 1

Given that, $\lambda =560\times {10}^{-9}m,\mu =1.4$
For strong reflection,
$2\mu d=(2n+1)\frac{\lambda }{2}\Rightarrow \mu =(2n+1)\left(\lambda \right)4d\Rightarrow d=(2n+1)\frac{1}{4\mu }$
For minimum thickness, putting n = 0
$\Rightarrow d=\frac{\lambda }{4d}\Rightarrow d=\frac{560\times {10}^{-6}}{14}={10}^{-7}m=100nm$