Question

A thin film of thickness i and index of refraction 1.33 coats a glass with index of refraction 1.50. What is the least thickness/that will strongly reflect light with wavelength 600 nm incident normally?

• A. 225 nm
• B. 300 nm
• C. 400 nm
• D. 450 nm

Answer 1

The correct option is A 225 nm

Coating of redfractive index 1.33 act as thin film interference.

$2\mu t=\frac{(2n-1)}{2\lambda },n=1,2,3,\dots$

$\therefore 2\mu t_{min}=\lambda$ or $t_{min}=\frac{\lambda }{2\mu }$

Given: $\lambda =600nm$ and $\mu =1.33$

$\therefore t_{min}=\frac{600nm}{2\times 1.33}=225nm$

Hence, minimum thickness of film is $225nm$