Optics 2 Question 5

5. A point source S is placed at the bottom of a transparent block of height 10mm and refractive index 2.72 . It is immersed in a lower refractive index liquid as shown in the figure. It is found that the light emerging from the block to the liquid forms a circular bright spot of diameter 11.54mm on the top of the block. The refractive index of the liquid is

(2014 Adv.)

(a) 1.21

(b) 1.30

(c) 1.36

(d) 1.42

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Answer:

Correct Answer: 5. (c)

Solution:

  1. At point Q angle of incidence is critical angle θC, where

sinθC=μlμblock 

In PQS,sinθC=rr2+h2

μlμblock =rr2+h2

μl=rr2+h2×2.72

=5.7711.54×2.72=1.36



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