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NDA 2025 GAT · General Knowledge · Light and Optics · medium

The refractive index of glass is 1.5. If the speed of light in vacuum is 3 x 10^8 m/s, the speed of light in glass is

Single correct option

The refractive index of glass is 1.5. If the speed of light in vacuum is 3 x 10^8 m/s, the speed of light in glass is
  1. A. 4.5 x 10^8 m/s
  2. B. 1.5 x 10^8 m/s
  3. C. 2 x 10^8 m/s
  4. D. 3 x 10^8 m/s
Show answer and solution

Answer: (C) 2 x 10^8 m/s

Refractive index n = c/v, so v = c/n = 3 x 10^8 / 1.5 = 2 x 10^8 m/s. Light slows down in a denser medium, so the speed must be less than c; this rules out 4.5 x 10^8 m/s, which comes from multiplying instead of dividing. The value 1.5 x 10^8 m/s comes from dividing by 2 instead of 1.5.
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Original question written in the NDA 2025 GAT pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake