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

Asked in GUJCET 2019 · Snell's law, slabs and apparent depth

• (1) 2×10⁻⁸
• (2) 2×10⁻¹¹
• (3) 2×10⁻¹⁰
• (4) 2×10¹¹

Step-by-step solution

Why the other options are wrong

• (1) Reading the thickness as 4 m gives 4/(2×10⁸)=2×10⁻⁸ s. The slab is 4 cm, a hundred times thinner.
• (2) 2×10⁻¹¹ s is the answer for a slab 4 mm thick: (4×10⁻³)/(2×10⁸). Here the thickness is 4 cm.
• (4) 2×10¹¹ s is about six thousand years — the minus sign in the exponent has been lost. Dividing 4×10⁻² m by 2×10⁸ m s⁻¹ can only give a negative power of ten.

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