Question

A glass prism of refractive index 1.5 is immersed in water (refractive index 4/3). A light beam incident normally on the face AB is totally reflected to reach the face BC if

• A. Sinθ > 8/9
• B. 2/3 < sinθ < 8/9
• C. Sinθ < 2/3
• D. None of the above

Answer 1

The correct option is A

Sinθ > 8/9

Let ${\theta }_c$ be the critical angle at face $BC,$ then

$\sin {\theta }_c=\frac{{\mu }_W}{{\mu }_g}=\frac{4/3}{3/2}=\frac{8}{9}$


Angle of incidence at face $BC$ is $i=\theta$.

Total internal reflection(TIR) will take place on this surface if,

$i>{\theta }_c$

Or $\theta >{\theta }_c$

Or $\sin \theta >\sin {\theta }_c$

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