Question

Time taken by sunlight to pass through a window
of thickness 4 mm whose refractive index is $\frac{3}{2}$ is

• A. $ 2 \times {10}^{-4} s$
• B. $ 2 \times {10}^{8} s$
• C. $ 2 \times {10}^{-11} s$
• D. $ 2 \times {10}^{11} s$

Answer 1

Answer: (C)

$v_g = \frac{ c }{\mu } = \frac{3 \times {10}^8}{3} = 2 \times {10}^ 8 m / s $
$ t = \frac{ x }{v_g} = \frac{ 4 \times {10} ^{-3}}{ 2 \times {10}^{8}} = 2\times {10}^{-11} s $