# Time required to cross 4 mm thick glass (μ = 1.5) for sunlight?

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Time required to cross 4 mm thick glass (μ = 1.5) for sunlight?
1. 4 × 10-11 sec
2. 3 × 10-11 sec
3. 2 × 10-11 sec
4. 2 × 10-8 sec

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Correct Answer - Option 3 : 2 × 10-11 sec

CONCEPT:

Refractive Index:

• The ratio of the speed of light in a vacuum and the speed of light in the lens material is called the Refractive Index.

$μ=\frac{c}{v}$ or $v=\frac{c}{μ}$

where c is the speed of light in a vacuum, v is the speed of light in a medium.

CALCULATION:

Given:

The thickness of the slab = 4 mm = 4 × 10-3 m; refractive index μ = 1.5

• Time to pass through the glass slab:

⇒ $t = {Distance\space travelled \over speed \space inside\space slab}= {Distance\space travelled \over \frac{c}{\mu}}$

⇒ $t = {4\times 10^{-3} \over \frac{3\times10^8}{1.5}}={4\times 10^{-3} \times1.5\over 3\times10^8}=2\times 10^{-11}sec$

So, option 3rd is correct.