Light travels slower in glass than in air because :

The speed of light in a medium is inversely proportional to its refractive index, as defined by the formula n = c/v, where 'n' is the refractive index, 'c' is the speed of light in a vacuum, and 'v' is the speed in the medium [c2][t2][t8]. Light travels slower in glass than in air because glass has a higher refractive index (approximately 1.50) compared to air, which has a refractive index marginally greater than 1.00 [c1][c2][t10]. Consequently, the refractive index of air is less than that of glass [t1]. While glass is physically denser than air, optical density (refractive index) is the determining factor for light speed, and mass density is not the same as optical density [c3][t6]. Therefore, the reduction in speed is fundamentally attributed to the higher refractive index of glass relative to air [t1][t7].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > QUESTIONS > p. 150
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.2 The Refractive Index > p. 148
3. [3] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > Activity 9.10 > p. 149