Why does light refract when entering a different medium of propagation?

When the light travels through other medium than air, the light travels at different speed from the speed of light. The speed of the light in material is related to index of refraction.

Hence, light have different speeds in different medium and also, will have different wavelength in different mediums.

The change in speed which occurs when the light propagates from one medium to the another is responsible for bending of the light which is known as refraction.