Question

A light ray does not bend at the boundary in passing from one medium to the other medium if the angle of incidence is:
A. $0^\circ$
B. $45^\circ$
C. $60^\circ$
D. $90^\circ$

Answer 1

We use here the concept of refraction which is the phenomenon of change in the direction of the path of light when it goes from one medium to another medium. The refraction of light occurs because the speed of light changes when it goes from one medium to another medium.

Complete step by step solution:
We know about TIR in which a ray travels from an optically denser medium to an optically rare medium at a certain angle it gets internal reflected and does not undergo refraction at all, it must be incident at an angle of zero degrees i.e. normally to the boundary. However, still it undergoes refraction as this will change the speed of light but no change in direction of the transmitted ray. Sometimes it is said that refraction takes place only when the ray changes while entering the second medium. That’s not true. The other factor associated is the change in velocity of light also.
We can calculate angle of refraction using snell’s law
${{\text{n}}_1}\sin {\theta _1} = {{\text{n}}_2}\sin {\theta _2}$
${{\text{n}}_1}$ = refractive index of medium 1
${{\text{n}}_2}$ = refractive index of medium 2
But ${{\text{n}}_1} \ne {{\text{n}}_2}$
Therefore Snell’s law equation with no refraction
${\text{sin}}{\theta _1} = {\text{sin}}{\theta _2} = 0$
$\therefore {\theta _1} = {\theta _2} = 0$
So normal incidence with zero angle of incidence, the angle of refraction is zero, but the speed of light is changed.

$\therefore$ Hence, Option (C) is correct.

Note:
Here snell’s law is not applicable as a refractive index of medium 1 w.r.t medium 2 for a ray going from one medium to medium $2 =\dfrac{\text{sin i}}{\text{sin r}}$ does not yield as angle $i =0$ here.