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Q99 (NDA-II/2020) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Geometrical optics

The refractive index of fused artz is 1.46 and that of sapphire is 1.77. If Vis the speed of light in artz and vs is the speed of light in sapphire, then which one of the following relations is correct ?

Result
Your answer: —  Â·  Correct: A
Explanation

The refractive index (n) of a medium is defined as the ratio of the speed of light in a vacuum (c) to the speed of light in that medium (v), expressed by the formula n = c/v [1][3]. This relationship implies that the refractive index is inversely proportional to the speed of light in the material [1]. Consequently, a higher refractive index indicates a lower speed of light within that medium. In this case, fused quartz has a refractive index of 1.46, while sapphire has a higher refractive index of 1.77 [1]. Since the refractive index of fused quartz is lower than that of sapphire, the speed of light in fused quartz (vq) must be greater than the speed of light in sapphire (vs) [1]. Therefore, the correct relation is vq > vs.

Sources

  1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.2 The Refractive Index > p. 149
  2. [3] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > What you have learnt > p. 159
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