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Time taken by sunlight to pass through a window of thickness 4 mm whose refractive index is 3/2 is

1. 2 × 10⁻⁴ sec
2. 2 × 10⁸ sec
3. 2 × 10⁻¹¹ sec
4. 2 × 10¹¹ sec

Correct answer: 2 × 10⁻⁴ sec

Solution

The time taken by light to pass through a medium is given by t = d / v, where d is the thickness and v is the speed of light in the medium. The speed of light in the medium is v = c / μ, where c is the speed of light in a vacuum (3 × 10⁸ m/s) and μ is the refractive index. Substituting μ = 3/2 and d = 4 mm = 4 × 10⁻³ m, we get t = (4 × 10⁻³) / (3 × 10⁸ / (3/2)) = 2 × 10⁻⁴ sec.

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