Cause of Refraction : The speed of the light in different transparent substances is different. For example speed of light in air is 3×10⁸ m/s and that of in glass is 2×10⁸ m/s. It is clear that the two speeds of the light are different. When light enters from air to glass the speed of the light gets decreased at the junction of the two media. This change in speed of the light affects the speed the of the various segments of the beam of the light. This can be concluded by the image. This change in speed of the light causes refraction.
Therefore the refraction of light is due to the change in the speed of light on going from one medium to another.
Now it is good question why change in speed causes refraction. Here is the answer. If we look the image carefully. The left part of the beam of light enters first into another medium while its right part remains still in first medium here air. Therefore the speed of the left part of the beam experiences speed reduction before that of the right part of the beam. This causes less distance covered by left side beam and more distance traveled by right side beam.
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