Question

A glass prism of refractive index 1.5 is immersed in water of refractive index 4/3. A light ray incident normally on face AB is totally reflected at face AC if

• A. $\sin \theta >8/9$
• B. $\sin \theta <2/3$
• C. $\sin \theta =\sqrt{3}/2$
• D. $2/3<\sin \theta \le 8/9$

Answer 1

The correct option is A $\sin \theta >8/9$

For total internal reflection to occur the angle of incidence should be more than the critical angle.

$\Rightarrow \sin {\theta }_c=\frac{4/3}{1.5}=\frac{8}{9}$ [Critical angle, ${\theta }_c$]

$\Rightarrow \sin \theta >\frac{8}{9}$ [Angle of incidence, $\theta$].