Tuesday, December 30, 2014

special relativity - How does wind speed affect the velocity of light?


As you know, there is a maximum speed things can go called $c$, the "speed of light." Light in a vacuum goes $c$. Light in the atmosphere, however, goes a little less than $c$.


My question is: what effect does wind have on light's velocity? Simply adding the wind's velocity to light's would not even be remotely close, since a 10 mph tail wind would probably push it over $c$.



Answer




If the air is not moving, we know the light moves at a speed $v=c/n$, where $n$ in the index of refraction of the air. Now if the air is moving at a speed $u$ relative to you, and the light is propagating in the same direction, then you can find the apparent speed of light by the relative addition of velocities formula. In this case, you will find the apparent speed of light to be $$\frac{v+u}{1+\dfrac{uv}{c^2}}. $$


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