A point source of light is placed 4 m below the surface

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A point source of light is placed 4 m below the surface of water of refractive index 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

1. ∞
2. 6 m
3. 4 m
4. 3 m

Correct Option: B

sin C =	1	=	1			=	3	⇒ tan C =	3
	μ			5			5		4
				3

Now, tan C =	r	; r = h tan C = 4 ×	3	= 3 m
	h		4

Diameter of disc = 2r = 6 m