Question

A thin film of refractive index $\frac{3}{2}$ and thickness $t$ is coated on surface of spectacles of refractive index 1.7, so as to save eyes from harmful light of wavelength 400 nm. Which of the following cannot be proper $t$ for this to happen?

• A. 400 nm
• B. 800 nm
• C. 200 nm
• D. $\frac{400}{3}nm$

Answer 1

The correct option is C 200 nm


For this to happen there should be distructive interference of transmitted light of $\lambda =400nm$

$2\mu t=n\lambda \Rightarrow t=\frac{n\lambda }{2\mu }=\frac{n\lambda }{\text{/}2\times \frac{3}{\text{/}2}}$

$t=\frac{n\times 400}{3}nm$