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A beam of light is travelling through space when it encounters and hits a glass of water. Hitting the glass and the water causes the beam to slow down to some velocity less than the speed of light. When the beam leaves the glass of water and reenters space, it is observed to again is travelling the speed the speed againtravelling the speed of light! travelling its original velocity, the speed of light! How can this happen?

It is wrong to think in terms of acceleration for light. Mainly because it is massless. Light is an electromagnetic wave, and its speed depends only on the way em fields behave in the given medium.

jasjeet s bagla

It should also be noted that even in water or glass, there is a very small "evanescent" component of the light wave that continues on ahead, travelling at the speed of light, sort of making way for the regular light wave coming behind at the slower speed of light for that medium.

Arthur Smith

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