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. sin $\theta > 8/9$
• B. 2/3 < sin $\theta$ < 8/9
• C. sin $\theta$ < 2/3
• D. None of these

Answer 1

Answer: (A)

sin $\theta > 8/9$

Answer 2

Let $\theta_c$ C. be the critical angle at face BC, then
\hspace15mm sin\theta_c=\frac{\mu_w}{\mu_g}=\frac{4/3}{3/2}=\frac{8}{9}
Angle of incidence at face B C is i =$\theta$
Total internal reflection (TIR) will take place on this surface if,
$i > \theta_c\, or\, \theta > \theta_c\, or\, sin\, \theta_c\, or\, sin\, \theta > \frac{8}{9}$
$\therefore$ Correct option is (a).