Question

Find the time taken by light to cover a distance of 9 mm in water? Take ${\mu }_{water}$ = $\frac{4}{3}$

• A. 0.4 ns
• B. 4 ns
• C. 400 ns
• D. 0.04 ns

Answer 1

The correct option is D 0.04 ns

Using snell's law,
${\mu }_{air}{v}_{air}={\mu }_{water}v$
$1\times 3\times {10}^8=\frac{4}{3}\times v$
$v=\frac{9}{4}\times {10}^{8}m/s$

From definition of speed,
$v=\frac{d}{t}$
$t=\frac{d}{v}$
$t=\frac{9\times {10}^{-3}m}{\frac{9}{4}\times {10}^{8}m/s}$
$t=4\times {10}^{-11}s$
$t=0.04ns$