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Time taken by the sunlight to pass through a slab of 4 cm and refractive index 1.5 is ______ s.

Asked in GUJCET 2017 · Snell's law, slabs and apparent depth

• (1) 2×10¹⁰
• (2) 2×10⁻⁸
• (3) 2×10⁸
• (4) 2×10⁻¹⁰

Step-by-step solution

Why the other options are wrong

• (1) The sign of the exponent has been dropped and the answer comes out twenty orders of magnitude too large. A few centimetres divided by 2×10⁸ m s⁻¹ must be a very small time.
• (2) This is the answer for a slab 4 m thick: 4/(2×10⁸)=2×10⁻⁸ s. The slab here is 4 cm, a hundred times thinner.
• (3) 2×10⁸ is the speed of light inside the slab in m s⁻¹, not the time. The thickness still has to be divided by it.

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