If the absolute refractive indices of glass and water are 3/2 and 4/3 respectively, what will be the ratio of velocity of light in glass and water ?

The absolute 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]. This relationship implies that the velocity of light in a medium is inversely proportional to its refractive index (v ∝ 1/n) [2]. Given the absolute refractive index of glass (ng) is 3/2 and water (nw) is 4/3, the ratio of the velocity of light in glass (vg) to the velocity in water (vw) is calculated as vg/vw = nw/ng [2]. Substituting the given values, the ratio becomes (4/3) / (3/2). Mathematically, this simplifies to (4/3) × (2/3) = 8/9 [2]. Therefore, the ratio of the velocity of light in glass to that in water is 8:9.

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