# Is classical Snell's law with a moving medium solvable?

• jk22
In summary, using a Lorentz boost, one can account for the material response and obtain a more accurate relativistic formula for Snell's law.
jk22
Considering classically a light ray in a medium with lightspeed ##c_1## and entering a medium with lightspeed ##c_2## moving with speed ##v## along let say ##y## in the plane of the interface, is it correct to obtain a very complicated formula, having even 3rd power of trigonometric functions of the refracted angles ?

I looked on Google but it's 2D and uses a Lorentz boost wheteas I would consider it classically.

I think you will have to use Lorentz boosts. You need this to account for material response. Here is why. In a simplified picture, light induces magnetization ##\vec{M}##, and polarization ##\vec{P}## in the medium it propagates. We usually observe it in a low-energy limit, where the material response is linear so one can introduce electric and magnetic susceptibilities ##\chi_e, \chi_m## and link the polarization/magnetization to electric ##\vec{E}## and magnetic ##\vec{H}##, as follows: ##\vec{P}=\chi_e \vec{E}##, ##\vec{M}=\chi_m \vec{H}##. This then trickles down into your refractive index ##n=\sqrt{\epsilon \mu}=\sqrt{(1+\chi_e)(1+\chi_m)}##.

Now let's return to what, for example, polarization is. It is a volume density of induced electric dipoles. But electric dipole only appears as such if it is stationary. If you have a pair of separated opposite charges (i.e. electric dipole) moving relative to you, you will observe it as electric & magnetic dipoles. This is due to Lorentz boosts. This also applies to polarization and magnetization, but even more so since they are densities (so there are additionnal transformations). Basically if you see a material with polarization moving, it will appear to have both polarization and magnetization. This will affect the susceptibilities and, in turn, the refractive index etc.

So, I think, you have to use Lorentz boosts.

This complicate my problem, I would like to see it as a particle moving with those speeds, and find the quickest path. But in the second medium the classical velocities addition ##\vec{v_2}+\vec{v}## were used instead of the relativistic one.

If it's a particle, why is it refracting? It needs to be a wave and needs to be interacting with the medium.

The usual way to do non-relativistic approximation is to Taylor expand the relativistic expression in ##v## and see under what circumstances (if any) you can neglect higher order terms. The last time you asked a similar question PAllen linked to a mathpages page that gives the exact relativistic version of Snell's Law - expand it and see if it matches what you got.

I'll just note that Fizeau's experiment was initially interpreted as supporting partial ether dragging because naive Galilean velocity addition did not match his results. So you may find that the domain of validity of your approximation is extremely narrow.

Cryo
Indeed. So the problem statement should be rather put as a vehicle with different speeds, at a classical level.

jk22 said:
at a classical level

You keep saying 'classical'='not relativistic', but there is no non-relativistic theory of electromagnetism. You can do full relativistic treatment and then take the limit of low velocity, sure, but you still have to go through relativity first. Maxwell's equations is what is commonly understood to be classical electromagnetism, and they are not invariant under Gallilei transforms, so there is no sane way to do electromagnetism 'non-relativistically'.

Ibix said:

If you are not considering light, then an example like a lifeguard trying to reach a person in trouble in water is a valid application for this, I think. In that case I presume you can do it in the same way as the usual still water example - fix the end points with respect to their media, parameterise possible paths by the point the lifeguard enters the water, and extremise.

I would expect this to be the low v, extremely high n (in both media), limit of the relativistic formula.

jk22 said:

This problem may be trivial when v << c, but I don't think the general problem is simple at all. My go-to reference for this is:

Electrodynamics of Moving Media
Paul Penfield, Hermann A. Haus
M.I.T. Press, 1967 - Electrodynamics - 276 pages

In it, there is a claim that using the Minkowski formulation correctly predicts Snell's law (and Cerenkov radiation), but as I said, the topic is highly non-trivial.

It is possible. For future searchers seeking an answer to this question, check out the article by C.K. Thornhill titled "The Refraction of Light in Stationary and Moving Refractive Media." There are several versions freely available. Get the 16 page version with helpful diagrams. He uses a Newtonian framework, wave model of light moving in a fluid medium.

jk22 said:
Considering classically a light ray in a medium with lightspeed ##c_1## and entering a medium with lightspeed ##c_2## moving with speed ##v## along let say ##y## in the plane of the interface, is it correct to obtain a very complicated formula, having even 3rd power of trigonometric functions of the refracted angles ?

I looked on Google but it's 2D and uses a Lorentz boost wheteas I would consider it classically.

berkeman
A google search reveals that this "paper" appeared on viXra and thus should be taken with a great portion of skeptical doubt. I don't read papers "published" there, because it's a waste of time. This you can see already from the few remarks you made above: Any attempt to describe light non-relativsitically is doomed to fail, because massless fields make no sense in any Newtonian framework.

## 1. What is Snell's law with a moving medium?

Snell's law with a moving medium is a variation of the classic Snell's law, which describes the relationship between the angle of incidence and the angle of refraction when light passes through a boundary between two different mediums. In this case, one of the mediums is in motion, meaning that it has a certain velocity.

## 2. Is classical Snell's law with a moving medium solvable?

Yes, classical Snell's law with a moving medium is solvable. It follows the same principles as the classic Snell's law, and the velocity of the moving medium can be incorporated into the calculations to determine the angle of refraction.

## 3. How is classical Snell's law with a moving medium different from the classic Snell's law?

The main difference between the two is the presence of a moving medium in the former. This means that the velocity of the medium must be taken into account in the calculations, whereas it is not a factor in the classic Snell's law.

## 4. Can classical Snell's law with a moving medium be applied to all types of mediums?

Yes, classical Snell's law with a moving medium can be applied to any type of medium, as long as the velocity of the medium is known and can be incorporated into the calculations. This includes both transparent and opaque mediums.

## 5. What are some real-life applications of classical Snell's law with a moving medium?

One common application is in the field of optics, where this law can be used to calculate the path of light through different mediums, such as in lenses or prisms. It is also used in meteorology to explain the bending of light as it passes through different layers of the earth's atmosphere.

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