Question

Time taken by sunlight to penetrate $2\ \text{mm}$ through a glass slab is of the order $\left[\text{Refractive index of glass} = 1.5,\ \text{velocity of light in air} = 3 \times 10^8\ \text{m/s}\right]$

• A. $10^{-19}\ \text{s}$
• B. $10^{-11}\ \text{s}$
• C. $10^{5}\ \text{s}$
• D. $10^{-10}\ \text{s}$

Hint:

Time taken by light depends on speed in the medium, not speed in vacuum.

Answer 1

The Correct Option is B

Step 1: Find speed of light in glass.
\[ v = \dfrac{c}{\mu} = \dfrac{3 \times 10^8}{1.5} = 2 \times 10^8\ \text{m/s} \] Step 2: Convert thickness into meters.
\[ 2\ \text{mm} = 2 \times 10^{-3}\ \text{m} \] Step 3: Calculate time taken.
\[ t = \dfrac{d}{v} = \dfrac{2 \times 10^{-3}}{2 \times 10^8} \] Step 4: Simplify.
\[ t = 10^{-11}\ \text{s} \]