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Refraction is the optical phenomenon by which a ray that enters from one material into another is bent from its original path.
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And this happens because the speed of light actually changes.
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You may have heard the speed of light is the same for electromagnetic waves and all forms of light.
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And that is true only in a vacuum.
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As soon as the light goes into a denser material than a vacuum, which is almost anything, it will slow down and can veer from a straight path if it comes in at an angle.
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The law that governs this bending is called snell's law and involves the two indices of refraction on either side of the material.
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You usually call it n1 on the incoming side, incident side, and the angle that you come in on must be made relative to the normal to the surface.
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So notice that over there.
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We're drawing a perpendicular to the surface of the material.
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And that has to equal the index on the other side that you're going into times the sign of the angle going into.
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And a reminder that index of refraction is coming from the speed of light in a vacuum, divided by the speed in the material v.
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So that is always slower.
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And the index, excuse me, the index is always bigger than one...