Optics 4 Question 14
14. A glass prism of refractive index 1.5 is immersed in water (refractive index 4/3). A light beam incident normally on the face $A B$ is totally reflected to reach the face $B C$ if
(1981, 3M)
(a) $\sin \theta>8 / 9$
(b) $2 / 3<\sin \theta<8 / 9$
(c) $\sin \theta<2 / 3$
(d) None of these
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Answer:
Correct Answer: 14. (a)
Solution:
- Let $\theta _c$ be the critical angle at face $B C$, then
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 $\sin \theta>\sin \theta _c$ or $\sin \theta>\frac{8}{9}$
