NEETPhysicsGeometrical OpticsMCQ+4 / −1
A disc is placed on a surface of pond which has refractive index 5/3. A source of light is placed 4 m below the surface of liquid. The minimum radius of disc needed so that light is not coming out is,
- A
- B3 m
- C6 m
- D4 m
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Correct answer: B

$$\theta = {\sin ^{ - 1}}\left( {{1 \over \mu }} \right)$$ = $${\sin ^{ - 1}}\left( {{3 \over 5}} \right)$$
$ \Rightarrow $ $$\sin \theta = {3 \over 5}$$
$ \therefore $ tan$\theta $ = ${3 \over 4}$ = ${r \over 4}$
$ \Rightarrow $ r = 3 m
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