First slide

Total internal reflection - Ray optics and optical instruments

Question

A point source of light is placed 4 m below the surface of water of refractive index $\frac{5}{3}$ . The minimum diameter of a disc which should be placed over the source on the surface of water to cut–off all light coming out of water is

Moderate

A

2 m
B

6 m
C

4 m
D

3 m

Solution

$Here sini = \frac{1}{μ} = \frac{3}{5} and hence tani = \frac{3}{4} = \frac{r}{4} This gives r = 3 m, hence diameter = 6 m$

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