Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

Why is it that mechanical waves/longitudinal waves/sound travel faster in a denser/stiffer medium as in steel compared to say air, while EM waves/trasverse waves/light travels slower in a (optically) denser medium as in say glass?

I know that for mechanical waves,$$v \varpropto \sqrt{T}$$, where $T$ stands for Tension

and $$c \varpropto \sqrt{\frac{1}{\epsilon_0\mu_0}}$$

Any physical interpretation might be helpful.

EDIT: What I understand from this is that, since higher tension implies stronger intermolecular interaction, which leads to better conveyance of energy across the material which causes the waves to move faster. But this analogy doesn't fit at all for light waves. Why?

share|improve this question
    
So, do You ask about density or "Tension" (what ever You mean with it) –  Georg Nov 30 '11 at 9:17
    
tension was for a 1D wave in case of a string. for general 3D waves, the formula involves Bulk modulus. en.wikipedia.org/wiki/Speed_of_sound#Basic_formula –  Vineet Menon Nov 30 '11 at 9:23
    
This "answer" is pure driveling. –  Georg Nov 30 '11 at 9:51

2 Answers 2

up vote 2 down vote accepted

The reason is that EM weves don't really propagate through matter -- they exists in vacuum and go with c there, and they interact with matter by being absorbed and re-emitted by the particles of the medium. This only looks like they were slowed down from the macroscopic perspective.

Mechanical weaves exist as a form of particle motion, and their speed roughly depends on how easily the distortion can propagate from one particle to another.
If you look at the weave equation, the speed is a square root of the stuff multiplying spatial derivative (connected with the strength of particle motion coupling) divided by the stuff multiplying temporal derivative (connected with the inertia of particles). How this is connected to the density depends on the medium and weave type.

share|improve this answer

Mechanical waves are slower in a denser medium, provided that you keep the stiffness of the material the same. Steel is about $10^3$ times more dense than air, but its resistance to compression is millions of times greater.

If your T means temperature, then $v \propto \sqrt{T}$ is not for mechanical waves in general, it's for gases. At fixed pressure, a higher temperature gives a lower density, which causes a higher v.

Electromagnetic waves are not really analogous to mechanical ones.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.