refraction is the change in direction of a wave passing from one medium to another or from a gradual change in the medium.

Refraction of light is the most commonly observed phenomenon, but other waves such as sound waves and water waves also experience refraction.

How much a wave is refracted is determined by 
1. the change in wave speed 
2. the initial direction of wave propagation relative to the direction of change in speed.

For light, refraction follows Snell's law,

Snell's law (also known as Snell–Descartes law and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass, or air.

Light slows as it travels through a medium other than vacuum (such as air, glass or water).

This is not because of scattering or absorption

Rather it is because, as an electromagnetic oscillation (the waves (or their quantaphotons) of the electromagnetic field, propagating (radiating) through spac), light itself causes other electrically charged particles such as electrons, to oscillate. 

The oscillating electrons emit their own electromagnetic waves which interact with the original light.

When light enters, exits or changes the medium it travels in, at an angle, one side or the other of the wavefront is slowed before the other. 

the speed of light is slower in a medium other than vacuum. This slowing applies to any medium such as air, water, or glass, and is responsible for phenomena such as refraction.

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