Question

A ray of light is incident on a thin film. As shown in the figure, $M$ and $N$ are two reflected rays while $P$ and $Q$ are two transmitted rays. Rays $N$ and $Q$ undergo a phase change of $\pi$. correct ordering of the refracting indices is

• A. $n_2>n_3>n_1$
• B. $n_3>n_2>n_1$
• C. $n_3>n_1>n_2$
• D. none of these, the specified changes cannot occur

Answer 1

The correct option is B $n_3>n_2>n_1$

Concept: 1) Transmitted ray don't undergo any phase change.

2) A ray reflected from a relatively denser medium undergoes a phase change of $\pi$

3) A ray reflected from a relatively rarer medium undergo any phase change.

Now as ray $N$ undergoes a phase change of $\pi$ w.r.t to incident ray, $⟹n_2<n_3$

As ray $Q$ undergoes a phase change of $\pi$ w.r.t to incident ray, $⟹n_1<n_2$

Thus $n_3>n_2>n_1$