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JEE Main 2025 (7 Apr, Shift 1) · Spherical Surfaces & Thin Lenses

Spherical Surfaces & Thin Lenses question from JEE Main 2025 (7 Apr, Shift 1)

Q20JEE Main 2025 · 7 APR · SHIFT 1Easy

Spherical Surfaces & Thin Lenses

A lens having refractive index 1.6 has focal length of 12 cm, when it is in air. Find the focal length of the lens when it is placed in water. (Take refractive index of water as 1.28)

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Answer: (c)

  1. fwfa=μ−1μμw−1\frac{f_w}{f_a} = \frac{\mu - 1}{\frac{\mu}{\mu_w} - 1}
  2. μμw=1.61.28=1.25\frac{\mu}{\mu_w} = \frac{1.6}{1.28} = 1.25, so fw12=0.60.25=2.4\frac{f_w}{12} = \frac{0.6}{0.25} = 2.4
  3. fw=28.8 cm=288 mmf_w = 28.8\text{ cm} = 288\text{ mm}
  4. So the answer is (c) 288 mm.
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