Question

A $600nm$ light is perpendicularly incident on a soap film suspended in air. The film is $1.00\mu m$ thick with $n=1.35$. Which statement most accurately describes the interference of light reflected by the two surfaces of the film?

• A. The waves are close to destructive interference
• B. The waves are close to constructive interference
• C. The waves show complete destructive interference
• D. The waves show complete constructive interference

Answer 1

Answer: (D)

The waves show complete constructive interference

As reflection from the thin film suspended in air is a case when the waves are out of phase (${180}^o$ out) to each other,

Given : $n=1.35t=\mu m\lambda =600nm$

thus using $2t=(m+\frac{1}{2})\frac{\lambda }{n}$ for constructive interference, gives integral value of $m(=4)$ .

It proves that waves show complete constructive interference.