# Doppler effect of light when it's windy

I think I understand the classical doppler effect in sound, where the equation is non-symmetric whether the source of the observer is moving because the speed of medium where sound wave propagates is different according to each of the observers.

I think I also understand why doppler effect is symmetric with light since the speed of "the medium" where light propagates is the same for both observers, meaning we need special relativity to explain the doppler effect of EM waves in a vacuum.

But I struggling to make an eqution to describe the doppler effect of light in an actual realistic moving medium.

What is the frequency shift of light between the source and the observer if wind is blowning at 1/3 of $$c_0$$, flowing towards the observer.

I have to somehow take in to effect the slowdown of light, the lenght contraction of space as well as the fact that for two observers, the light is now travelling at different speeds. The source is here glowing his laser beem in a lenght-contracted medium. It gets even stranger if you change the wind to water and assume the water is moving faster than the speed of light in water.

On a nano-level, the slowdown of light is caused by the delay in absorption and emmitance speeds of photons in. If the wind is blowing, it is moving those tiny photon-emmiting molecules in space thus causing a classical doppler shift as well.

• This article analyses the Doppler effect for sound and light in a single equation: mathpages.com/rr/s2-04/2-04.htm – m4r35n357 May 10 at 10:50
• What do mean by $\frac{1}{3}$ of cO(maybe you mean $c_0$, the speed of light in vacuum??) – PNS May 10 at 11:24
• Yes, speed of light in vacuum. I will go through math in m4r35n357's link and see if it's the answer I want. – KrNeki May 10 at 11:43
• en.wikipedia.org/wiki/Laser_Doppler_velocimetry (this is a practical way to measure the flow of liquids. I have no idea whether it would be practical for a fluid as tenuous as air.) – besmirched May 10 at 13:12
• FWIW, measuring the speed of light in moving water was historically important in the period leading up to the development of relativity. See en.wikipedia.org/wiki/Fizeau_experiment – PM 2Ring May 10 at 14:34

I have to somehow take in to effect the slowdown of light, the lenght contraction of space as well as the fact that for 2 observers, the light is now travelling at different speeds.

It is considerably easier than that. You simply use the relativistic velocity addition formula adding the speed of the medium in the frame of interest and the speed of light in the medium. So in your case the speed of the air is 0.3 c and the speed of light in air is 0.9997 c. Using the relativistic velocity addition formula we get:

$$\frac{u+v}{1+vu/c^2}=0.9998 c$$

I completely agree with Dale, but since the OP talked about both air and water, I decided to generalize the above answer further. The speed of light in any medium is given by $$v = \frac{c}{n}$$ where c is the speed of light in vacuum (the OP denoted this by $$c_0$$), and $$n$$ is the absolute refractive index of the medium. Doing a bit of algebraic 'juggling' we get $$w = \frac{u + v}{1 + uv/c^2} = \frac{u + c/n}{1 + \frac{u * c/n}{c^2}}$$$$= \frac{u + c/n}{1 + \frac{u}{nc}}$$

I know this looks a little messier, but you can just plug in the refractive index to get the final velocity in whichever medium you want.

• I like it with the refractive indexes since I can now think of a laser beam entering a spinning glass disc and that the angle of refraction now doesn't only depend on n, but on n and the angular speed of the glass and it's different depending on which direction it's going.. Wow. Now in have an equation that explains doppler when Emitior and Absorber still and wind is moving, and m4r35n357 gave me a link to where E and O are moving but the wind is still. I have to now formulate an equation where all 3 can be moving from my perspective. – KrNeki May 11 at 8:26
• Happy that you liked it! – PNS May 11 at 8:31
• Do the equation hold true for any n? Including those smaller than 1 (Ionosphere) and those with a negative n, in which cas should n be under an absolute value, or is that a field where it gets even more messy? Do these equations hold for any speed, even if speed of medium is faster than the speed of light in the medium? – KrNeki May 11 at 23:49

Users PNS and Dale contributed heavily towards answering a solution, but didn't actually answer the question which was "What is the frequency shift..."

I know now that I have to add speeds with the relativistic formula for speed addition when it comes to relativistic speeds, but I did not know to which Doppler equation I have to insert these speeds.

So here is what I've came up with:

Legend:

• $$c_s$$ - signal speed from my reference point
• $$c_{c_0}$$ - signal speed in medium
• $$v_w$$ - speed of medium (wind speed)
• $$v_e$$ - emittor speed
• $$v_a$$ - absorber speed
• $$f_e$$ - frequency emitted
• $$f_a$$ - frequency received by absorber
• speed of Emittor is positive when it is trying to approach the Absorber
• speed of Absorber is positive when it is trying to approach the Emmitor
• speed of the medium is positive when it s flowing from Emmitor to Absorber

The classical Doppler formula looks like this:

$$f_a=f_e {\frac{1-\frac{v_a}{c_s}}{1+\frac{v_e}{c_s}}}$$

Classical Doppler formula is usually used for sound, but it is also frequently used for radio-waves traveling through vacuum (although the equation is further simplified with the first order Taylor series).
If I also want to include the speed of wind relative to me, an outside observer, the formula looks like this:

$$f_a=f_e {\frac{1-\frac{v_a}{c_{s_0}+v_w}}{1+\frac{v_e}{c_{s_0}+v_w}}}$$

As long as $$v_w$$, $$v_a$$ and $$v_e$$ are are at non-relativistic speeds the formula is accurate enough. But it's not accurate anymore when one of those speeds approaches the relativistic speeds (such as $$\frac{1}3$$ the speed of light in vacuum).

So I thought I'd use the relativistic Doppler formula: $$f_a=f_e \sqrt{\frac{1-\frac{\Delta v}{c_0}}{1+\frac{\Delta v}{c_0}}}$$

Where $$\Delta v$$ is simply $$v_e+v_f$$ when $$v_e,v_f << c_0$$ and $$\Delta v = \frac{v_e+v_f}{1+\frac{v_ev_f}{c_0^2}}$$ otherwise.

The formulas would look like this

$$f_a=f_e \sqrt{\frac{1-\frac{v_e+v_f}{c_0}}{1+\frac{v_e+v_f}{c_0}}}, f_a=f_e \sqrt{\frac{1-\frac{\frac{v_e+v_f}{1+\frac{v_ev_f}{c_0^2}}}{c_0}}{1+\frac{\frac{v_e+v_f}{1+\frac{v_ev_f}{c_0^2}}}{c_0}}}$$

But how am I going to add the medium in it?

It is true, that if I just replace the $$c_0$$ with $$c_s$$, the accuracy of the relativistic Doppler formula increases, but lowering the $$c_s$$ in the relativistic formula brings another problem:

Relativistic Doppler formula is only accurate when all speeds are much smaller from the speed of light in the vacuum. And if I use the the $$c_0 --> c_s$$ hack, it's accuracy decreases as soon as any of the speeds come close to the speed of signal in the medium. $$v_e, v_a << c_s$$ must hold to use this equation.

Because my original question was about signal traveling in water, where light travels 25% slower than in vacuum, neither the Classical nor the Relativistic Doppler are good enough approximates because both are contributing to the end result. So I have to somehow use both.

Luckily I've come about this article which describes the general Doppler for any speed derived from relativistic Minowski diagram. It is exact for any speed as long as they are such that they don't produce any shockwaves. If they do, the frequency becomes negative, and that can either mean that the Absorber is moving too fast from the Emmitor for the signal to even reach him or that the Emmitor is traveling so fast in the medium that he would reverse the signal (and overtake it).

The general Doppler formula from mathpages.com looks like this:

$$f_a=f_e {\frac{1-\frac{v_a}{c_s}}{1+\frac{v_e}{c_s}}\sqrt{\frac{1-\frac{v_e}c}{1+\frac{v_e}c}}}$$

All I have to do now is modify it in a way to incorporate the change of signal speed due to the medium moving (wind blowing).

For non-relativistic speeds and any speed of the signal in medium: $$c_s = c_{s_0} + v_w$$

For any speeds, as long as they don't produce shockwaves: $$c_s = \frac{c_{s_0} + v_w}{1+\frac{v_w v_{s_0}}{c_0^2}}$$

And I get the the most general formula: $$f_a=f_e {\frac{1-\frac{v_a}{\frac{c_{s_0} + v_w}{1+\frac{v_w v_{s_0}}{c_0^2}}}}{1+\frac{v_e}{\frac{c_{s_0} + v_w}{1+\frac{v_w v_{s_0}}{c_0^2}}}}\sqrt{\frac{1-\frac{v_e}c}{1+\frac{v_e}c}}}$$

And because my question was about the frequency shift when both the observer and emitter are stationary, I enter zero for $$v_e$$ and $$v_e$$ and I get:

$$f_a=f_e {\frac{1-0}{1+0}\sqrt{\frac{1-0}{1+0}}} = f_e$$

There is no Doppler effect when the speeds of emittor and absorber are the same.

URL for Desmos graph to play with