# A Disc Is Placed On A Surface Of A Pond Which Has Refractive Index 5/3. A Source Of Light Is Placed 4 M Below The Surface Of A Liquid. Find The Minimum Radius Of A Disc So That Light Does Not Come Out From A Disc?

Given:

Refractive index = $$\frac{5}{3}$$

Distance = 4m

By using Pythagoras theorem,

$$sin\;c = \frac{x}{r^{2} + x^{2}} = \frac{1}{\mu} \\ \Rightarrow r = \frac{h}{\sqrt{\mu ^{2}-1}} \\ \Rightarrow \frac{4}{\sqrt{\frac{5}{4}}-1} \\ \Rightarrow \frac{4}{\sqrt{\frac{16}{9}}}\\ \Rightarrow\frac{4 * 3}{4}\\ \Rightarrow 3 m$$

Therefore, the minimum radius of a disc so that light does not come out from a disc is 3m.

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