The motion of light depends crucially on the material in which it is traveling. When light passes from one medium to another, an unexpected thing happens: the direction of travel changes. There are many explanations for why this happens and many of those explanations are wrong. In this video, Fermilab’s Dr. Don Lincoln explains the real reason.

Correction: Dr. Don is aware that the direction the plus and minus charges moved at 12:02 is backward. He apologizes.

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