How does wind speed affect the velocity of light?

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Solution 1

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 $$\frac{v+u}{1+\dfrac{uv}{c^2}}. $$

Solution 2

Two things:

  1. Medium motion does 'drag' speed of light. http://en.wikipedia.org/wiki/Light-dragging_effects
  2. Speeds of medium and c are not linearly added. Use Lorentz Transformation: http://en.wikipedia.org/wiki/Lorentz_transformation
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Updated on May 26, 2020

Comments

  • PyRulez
    PyRulez over 3 years

    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 $c$.

  • jinawee
    jinawee over 9 years
    Wouldn't turbulence and preassure be relevant for the refractive index?
  • Brian Moths
    Brian Moths over 9 years
    That's a good point. In reality wind is not just a uniform motion of air at a constant speed. The pressure and temperature will be non-uniform, and this will cause the index of refraction to be non-uniform. This effect on the speed of light is probably greater than the effect I talked aobut in my answer. However, it is not as easy to get a clean answer as to what the speed of light should be with these effects taken into account.
  • Reinstate Monica -- notmaynard
    Reinstate Monica -- notmaynard over 9 years
    If you could expand this answer to more than just two links, that would be helpful. Can you post the relevant info from those links here?
  • PyRulez
    PyRulez over 9 years
    Or you could just do math.
  • binaryfunt
    binaryfunt over 9 years
    Is there a reason why velocity addition applies to light in a moving medium?
  • Brian Moths
    Brian Moths over 9 years
    @BrianFunt I think I should have explained this better. Basically imagine you are standing on train tracks with the wind blowing at some speed $u$. Now you are wondering what the speed of light is. Luckily your friend is coming by on a train moving at the same speed $u$. You ask him what the speed of light is, and since he sees no wind the answer is easy: $v=c/n$. Now we have the light moving at a speed $v$ in a frame which is moving at a speed $u$ with respect to $u$, and we want to know what is the velocity in your frame. The asnwer is given by the relativistic velocity addition formula.
  • binaryfunt
    binaryfunt over 9 years
    Is it just coincidental that the group velocity of light is being treated similarly to an 'object' moving at speed $v$?
  • Brian Moths
    Brian Moths over 9 years
    @BrianFunt No that is not a coincidence. If you have a one dimensional set of space-time events described by $dx/dt = v$ in some coordinates, and then you used primed coordinate which are boosted by a speed $u$, then the $dx'/dt'$ will be equal to the boosted speed. This is just due to the geometry of space-time. It is true independently of whether the set of events is the worldline of a particle or the wavefront of light or just an arbitrarily chosen path through spacetime.
  • PyRulez
    PyRulez almost 3 years
    @binaryfunt the velocity formula applies to reference frames in general. There do not even need to be objects involved, only reference frames.