Q. The velocity of light in a medium is half its velocity in air. If ray of light emerges from such a medium into air, the angle of incidence, at which it will be totally internally reflected, is
A) $15{^\circ}$
B) $30{^\circ}$
C) $45{^\circ}$
D) $60^{o}$
Explanation
(b) $\mu = \frac{c}{v} \Rightarrow \mu = \frac{c}{c/2\ } = 2$ also for total internal reflection
$i > c \Rightarrow \sin i \geq \sin c \Rightarrow \sin i \geq \frac{1}{\mu}$
Hence $i \geq \sin^{- 1}\left( \frac{1}{\mu} \right)$ or $\mu \geq 30{^\circ}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.