Time taken by sunlight to pass through a window of thickness

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Time taken by sunlight to pass through a window of thickness 4 mm whose refractive index
is 3 is
2

1. 2 × 10^–4 sec
2. 2 × 10⁸ sec
3. 2 × 10^–11 sec
4. 2 × 10¹¹ sec

Correct Option: C

v_g =	c	=	3 × 10⁸			= 2 × 10⁸ m / s
	μ			3
				2

t =	x	=	4 × 10^-3	= 2 × 10^-11 s
	v_g		2 × 10⁸