Question

What is the minimum thickness of a thin film required for constructive interference it the reflected light from it ?
Given, the refractive index of the film = $1.5$, wavelength of the light incident on the film = $600\, nm$.

• A. 50 nm
• B. 200 nm
• C. 100 nm
• D. 300 nm

Answer 1

Answer: (C)

Condition for constructive interference is
$2 \mu t=[2 n+1] \frac{\lambda}{2}$
where, $\quad n=0,1,2,3...$
For minimum thickness, $n=0$
$2 \mu t=\frac{\lambda}{2}$
$\Rightarrow t =\frac{\lambda}{4 \mu}=\frac{600 \times 10^{-9}}{4 \times 1.5}$
$= 100\, nm$