A small coin is resting on the bottom of a beaker filled with liquid… | NEET 2007 (AIPMT)

Q56NEET 2007 · AIPMTMedium

Total Internal Reflection

A small coin is resting on the bottom of a beaker filled with liquid. A ray of light from the coin travels upto the surface of the liquid and moves along its surface. How fast is the light travelling in the liquid?

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Answer: (d)

The ray grazes the surface, so it meets the surface at the critical angle $C$ .
From the figure, horizontal 3 cm and vertical 4 cm, so the path length is 5 cm and $\sin C = \frac{3}{5}$ .
$n = \frac{1}{\sin C} = \frac{5}{3}$ .
$v = \frac{c}{n} = 3\times10^8 \times \frac{3}{5} = 1.8\times10^8$ m/s.
So the answer is (d) $1.8\times10^8$ m/s.

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Total Internal Reflection question from NEET 2007 (AIPMT)

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