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  4. If the velocity of light in vacuum is 3×108 ms-1, the time taken (in nanosecond) to travel through a glass plate of thickness 10 cm and refractive index 1.5 is

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Q. If the velocity of light in vacuum is $3\times10^{8}\, ms^{-1},$ the time taken (in nanosecond) to travel through a glass plate of thickness $10 \,cm$ and refractive index $1.5$ is

WBJEEWBJEE 2012Ray Optics and Optical Instruments

Solution:

Given $v=3 \times 10^{8} ms ^{-1}, d=10\, m$ and $\mu=1.5$
Here, $t=\frac{d}{c / \mu}$
$t=\frac{10 \times 10^{-2}}{2 \times 10^{8}}$
$\left(\therefore \frac{c}{\mu}=\frac{3 \times 10^{8}}{1.5}=2 \times 10^{8}\, ms ^{-1}\right)$
$t=0.5 \times 10^{-9} s$