Question

What is the time taken by light to cross a glass 2 mm thick? (Refractive index of glass = 1.5)

• A. ${10}^{-10}s$
• B. ${10}^{-12}s$
• C. ${10}^{-11}s$
• D. ${10}^{-9}s$

Answer 1

The correct option is C ${10}^{-11}s$

Whenever light goes from one medium to another, then velocity of light and the wavelength of light change.

Time taken by light to cross a glass is:

$t=\frac{\text{thickness of glass plate}}{\text{velocity of light}}$

$t=\frac{2\times {10}^{-3}}{\frac{3\times {10}^8}{1.5}}$ (As, $v=\frac{c}{\mu }$)

$=\frac{2\times {10}^{-3}\times 1.5}{3\times {10}^8}={10}^{-11}s$.