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Q & A: Velocity of light in a moving medium

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Most recent answer: 04/22/2013
Q:
The speed of light in vacuum is always c, no matter which frame of reference we use. Changing speed of the light source to v has no effect too (i.e., speed of light will not be v+c). When light enter a medium with refractive index n, the speed of light slows down to (c/n). In this case, will we always measure c/v for the speed of the light in the medium regardless of which frame of reference we use? Will moving the light source with a speed of v in the medium affect c/n (i.e., change its speed to c/n+v)?
- tom (age 30)
Singapore
A:
This is an interesting question and had early physicists puzzled for a long time.   Eventually a French physicist, Fizeau, performed an experiment that demonstrated the effect. Ultimately, careful analysis of special relativity, coupled with a medium with an index of refraction, showed agreement.   Fizeau's experiment involved shining light down a tube filled with running water. He found  that  Vlight =  c/n + Vwater(1 - 1/n2),  which is the correct result when Vwater << c.

LeeH

(published on 04/22/2013)

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