Q. What is the minimum thickness of a thin film required for constructive interference in the reflected light from it? Given, the refractive index of the film = 1.5 Wavelength of the light incident on the film $= 60nm$
A) $100\ nm$
B) $300\ nm$
C) $50\ nm$
D) $200\ nm$
Explanation
(a) The condition for constructive interference in a thin film of thickness $t$ and refractive index $\mu$ in the reflected system, for normal incidence is
$2\mu t = \left( n + \frac{1}{2} \right)\lambda,where\ n = 0,\ 1,\ 2,\ 3$
For minimum thickness, $n = 0$
$\therefore 2\mu t = \frac{\lambda}{2} \Rightarrow t = \frac{\lambda}{4\mu} = \frac{600 \times 10^{- 9}}{4 \times 1.5} = 100nm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.