Question

A glass prism of $\mu =$ 1.5 is immersed in water of $[\mu =\frac{4}{3}]$ as shown in this figure. A ray of light incident normally on face AB is totally reflected at the face AC if $sin\theta$ is

• A. less than or equal to $\frac{2}{3}$
• B. greater than $\frac{8}{9}$
• C. $0.8666$
• D. between $\frac{2}{4}and\frac{8}{9}$

Answer 1

Answer: (B)

greater than $\frac{8}{9}$

Total internal reflection should be at point P, so that all the rays can be reflected.
For total internal reflection incident agnle on face AC should be grater than the crictical angle.

For crictical angle -

Apply snell's law:

${\mu }_{1}sin\theta ={\mu }_{2}sin90$

$⟹1.5sin\theta =\frac{4}{3}$

$⟹sin\theta =\frac{8}{9}$

For total internal reflection -

$sin\theta >\frac{8}{9}$

Hence, optionB is correct answer.