Author: Khan Academy via YouTube
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Light refracts—it bends—when it passes from one medium into another at an angle. Refraction occurs because the phase velocity of light is different in different media. For example, light slows down when it passes from air into glass. This happens because the incoming electromagnetic wave causes electrons in the medium to oscillate, which produces a secondary electromagnetic wave. The resulting wave, which is a superposition of the incoming wave and the secondary wave, has a different phase velocity than c.

Sections:
00:00 – What is refraction?
01:19 – Understanding wavefronts
03:35 – Why does light refract?
05:56 – What does bending of light depend on?
07:20 – Why does light change speed?
08:30 – Constructive and destructive interference
10:54 – Interference causes phase kick back
12:05 – Modeling change in light speed as a continuous phase kick back
13:35 – What causes dispersion?

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