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Weve been taught that when a beam of light passes into a block of glass at an angle it bends because of a difference in velocity across the beam. OK. So when that same beam passes out of the glass why does it bend again? For this to happen the leading adge of the beam must have accelerated. How can it be possible for the beam of light to increase velocity if no energy of any kind is applied?
Great question! You've obviously been taking these ideas to heart.
You can't really think of light as made of little classical
particles with some mass and with some energy that grows as they move
faster. The energy is stored in the electromagnetic field as the light
travels through a vacuum. As the light travels through glass, some of
the energy is stored in stretching and compressing the bonds between
the atoms of the glass, and the electrical and magnetic fields are a
bit smaller. When the light leaves the glass, that energy goes back
into the electromagnetic fields.
You may have noticed that I haaven't really explained why the light
travels at different speed in the glass and in a vacuum. Let us know if
you want us to try to explain that. Otherwise, beginning books on
electromagnetism tell about that.
(republished on 07/29/06)
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