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Q83 (NDA-II/2022) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Geometrical optics

The refractive index of crown glass is close to 3/ 2. If the speed of light in air is c, then the speed of light in the crown glass will be close to

Result
Your answer: —  Â·  Correct: C
Explanation

The refractive index (n) of a medium is defined as the ratio of the speed of light in a vacuum or air (c) to the speed of light in that specific medium (v) [1][2]. This relationship is mathematically expressed by the formula n = c / v. According to the question, the refractive index of crown glass is approximately 3/2. By substituting this value into the formula, we get 3/2 = c / v. To find the speed of light in the crown glass (v), we rearrange the equation to solve for v, which yields v = c / (3/2). This simplifies to v = (2/3)c [2]. Therefore, light travels at two-thirds the speed of light in air when passing through crown glass, as the material's optical density slows it down by a factor equal to the refractive index [1].

Sources

  1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.2 The Refractive Index > p. 148
  2. [2] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > What you have learnt > p. 159
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