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Q & A: light leaving glass

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Most recent answer: 10/22/2007
Q:
how can light gain its original speed in vaccuum after it passes through another medium e.g glass ?
- atimo (age 19)
univercity of khartoum, cairo, egypt
A:
That's a really nice question.

The odd thing is that even in a medium the basic electromagnetic speed c is not really changed. However, all those charged particles around are accelerated back and forth by the electromagnetic field, and re-radiate their own fields. The reason that light travels slower inside the medium involves a subtle cancellation between those fields and the bare field of the initial incoming wave. Once the wave is back out in the open, the only speed it can travel at, according to the wave equation (from Maxwell's equations), is c.

I bet you can also see something like this with water waves travelling through regions with different depths.

Mike W.

(published on 10/22/2007)

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