# How can light propagate in fluids when it is a transverse wave?

Transverse waves involve, as we all know, the displacement of particles of medium perpendicular to the direction of energy propagation. Thus, each element of the medium (in a very exaggerated, magnified picture) exerts shearing stress on the adjacent element. However, since fluids have no shear modulus to speak of, how is it that light (which is a transverse wave) propagates through water? Is it perhaps the electromagnetic character of light that permits this?

• I edited the title because this question is more about EM waves and not so much about transverse waves in fluids. Feel free to change it ito something else if you feel this title does not reflect your question. May 3 '20 at 11:11

Light as a wave is not the same as wave motion in a collection of particles. Light passing through a medium does not displace particles in the medium itself. That would be a sound wave.

Rather, light is a wave where rather than the particles in the medium moving, the field lines of the electric and magnetic field move and oscillate. So the problem of having versus not having a shear modulus is not relevant to light.

Light is a non-mechanical wave, which means its propagation has no relation to the oscillation of the material medium in which it is travelling.

• What does it even mean for something described by physics to be "non-mechanical"? Is it the same as "magical" haha? May 3 '20 at 11:28
• @Steve Which part of "... non-mechanical wave, which means its propagation has no relation to the oscillation of the material medium in which it is travelling." doesn't seem to define a mechanical wave?
– user258881
May 3 '20 at 11:31
• I'm mainly concerned about the inadequate articulation of what a non-mechanical wave is. Propagation must have some relation to the material medium (i.e. the water in this case), because travelling through water has effects like a change in the propagation speed and various optical effects. If the water had "no relation" then we would expect light to pass through totally transparently. May 3 '20 at 11:46
• @Steve Yes, you are right in saying that the material medium has an effect on the propagation of light, however "propagation has no relation to the oscillation of the material medium in which it is travelling." The reason why speed changes is due to the change in permeability and permittivity, both of which do not relate to the oscillation of the medium in any way. And I thought I was displaying brevity by writing this short answer, but maybe it isn't true :)
– user258881
May 3 '20 at 11:48
• @Steve This would really look good as an answer and that's why I urge you to write an answer to this question :) There's nothing wrong with writing an answer if other answers feel inadequate. In fact, you've suggested a different POV which shouldn't just be buried in the comments, rather it should be converted to an answer.
– user258881
May 3 '20 at 12:09

It is indeed the electromagnetic character of light that permits this. Light propagates even in a vacuum because of Maxwell's equations. It can interact with a medium causing its speed to change* but the propagation isn't caused by the medium.

*The speed of light changes in materials but the speed at which information travels is always $$c$$.

• "Information" is not a specific physical thing. There is no informational force - any more so than there is an energy force. I wouldn't confound the issue by introducing such terms without explanation. May 3 '20 at 13:31