Question

A parallel beam of light of the 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?

• A. 100 nm
• B. 200 nm
• C. 300 nm
• D. 400 nm

Answer 1

The correct option is A 100 nm

Given that, $\lambda =560\times {10}^{-9}m,\mu =1.4$
For strong reflection, $2\mu d=(2n+1)\frac{\lambda }{2}$
$\Rightarrow d=\frac{(2n+1)\lambda )}{4\mu }$
For minimum thickness, putting n = 0.
$\Rightarrow d=\frac{\lambda }{4\mu }\Rightarrow d=\frac{560\times {10}^{-9}}{5.6}={10}^{-7}m=100nm$