Refraction is a bending that occurs when a wave moves from one medium to another at an angle and changes speed.

Refraction is a bending that occur when wave moves from one medium to another, each being of a different density. it is the change in density of the medium which causes refraction or the bending of light to occur. if light is moving from a less dense medium to a more dense medium, then the angle of refraction will be less than angle of incidence, meaning that light will bend and move at a slower speed. on the other hand, the opposite will happen when light or other wave travels from a more dense medium to a less dense medium. 

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