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# light travels through a glass of refractive index $\mu =1.5$ of thickness $4 mm$. What is the time taken for the light to cross it

$(a)\;2 \times 10^{-11}\;seconds \\ (b)\;4 \times 10^{-11}\;seconds \\ (c)\;6 \times 10^{-11}\;seconds \\ (d)\;8 \times 10^{-11}\;seconds$

optical distance in a medium of refractive index $\mu$ and thickness t is $\mu x$
$\therefore$ time $=\large\frac{\mu x}{c}$ here $c$- is velocity of light
$t= \large\frac {1.5 \times 4 \times 10^{-3}}{3 \times 10^8}$
$\qquad= 2 \times 10^{11}\;seconds$
Hence a is the correct answer.

edited Jul 14, 2014