Question

A light ray enters a prism through the side PR. PQR is a cross section of the prism. The emergent ray lies along the boundary OQ. ∠P = 90° and PQ = PR. The ray diagram has been given.

iii. If we immerse the whole system fully under water (refractive index = 1.3), what will happen?

[1 mark]

• A. The ray follows the OQ path.
• B. We observe a total internal reflection.
• C. The angle of emergence > ∠BOC
• D. The angle of emergence < ∠BOC

Answer 1

The correct option is C The angle of emergence > ∠BOC

Writing Snell’s law for calculating the critical angle when the light ray goes from glass to water,

$\Rightarrow \frac{{\mu }_{glass}}{{\mu }_{water}}=\frac{1}{\sin C}\Rightarrow \sin C=\frac{{\mu }_{water}}{{\mu }_{glass}}=\frac{1.3}{1.41}$

This critical angle will be more than 45° from the given expression. Since the angle of incidence (∠BOC) is 45°, the total internal reflection will not happen. Hence, the ray will undergo refraction and the ray will move away from the normal since it is going from a denser medium to a rarer medium.

Therefore, the angle of emergence will be more than ∠BOC.