Refraction of light, as it enters from one transparent medium to another, is due to

Refraction is the phenomenon where a ray of light changes its direction when traveling obliquely from one transparent medium to another [1]. This change in direction occurs because light propagates with different speeds in different media [2]. Specifically, refraction is fundamentally due to the change in the speed of light as it enters a new medium [4]. The extent of this bending is expressed by the refractive index, which is defined as the ratio of the speed of light in a vacuum to its speed in the specific medium [2]. When light enters a medium with a different refractive index, its speed changes, causing the wavefront to bend according to Fermat's principle of least time. While factors like temperature can influence the refractive index of a material, the immediate physical cause of the refraction process itself is the variation in the speed of propagation [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