Question

A parallel beam of light of wavelength $560nm$ falls on a thin film of oil in air ( refractive index=$1.4$). Whaat should be the minimum thickness of the film so that it weakly transmit the light?

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

Answer 1

The correct option is A $100nm$

The condition for strong reflection is,

$2\mu t=(2n+1)\frac{\lambda }{2}$

$\Rightarrow t=(2n+1)\frac{\lambda }{4\mu }$

for minimum thickness, put $n=0$ we get,

$\Rightarrow t=\frac{\lambda }{4\mu }$

$=\frac{560\times {10}^{-9}}{4\times 1.4}$

$={10}^{-7}m=100nm$