What causes dispersion?

For light waves, dispersion can occur in many ways. Most basically, the refractive index of a material is usually a function of the wavelength of light passing through the medium. This difference in refractive index for different wavelengths leads to a difference in angle of refraction (and velocity), causing dispersion. This is what happens in a prism.

 

The index of refraction is a property of the material in question. The structure of the medium as well as the what kind of atoms it's made from are very important parameters that decide how the index of refraction will vary with frequency.

 

The "anomalous" dispersion is found in and around absorption bands.

Basically, if an oscillator is excited by a wave whose frequency is close to the resonant frequency of the oscillator but slightly lower, the oscillator follows the forcing wave but is behind the forcing wave.

If, however, the oscillator is excited by a wave whose frequency is close to the resonance but slightly higher, the oscillator gets ahead of the forcing wave.

The result is that if you look at light near but slightly redward of an absorption band, you would see unusually high refractive index. If you look at light near but slightly blueward of an absorption band, you would see unusually low refractive index.

Between absorption bands, there is "normal dispersion" - the refractive index increases blueward and does so slowly.

Most transparent colourless substances are between two groups of absorption bands - one in ultraviolet caused by electron excitations, and the other in infrared caused by vibrations of nuclei.

The reason substances have light speed less than in vacuum is that the electronic excitations in ultraviolet get polarized by light even at much lower frequencies, and slow down the light.

In some substances, electrons are notably tightly held - with the result of weak refraction and also weak dispersion. Such as fluorides - the absorption of fluorides is notably far in UV, their electrons have low polarizability, the fluorides have low refractive index and also weak dispersion (big Abbe numbers).

Is that more informative?

 

The fact is that we don't actually know what you don't know. That is not a very polite response. Try to be a bit nicer, if you can. It will elicit better responses, young man. I am correct about your gender, I think.

 

In an isotropic medium you can have sound waves with different polarizations: longitudinal and transverse. They don't have the same speed usually. These are the S and P waves (so named in geophysics). In anisotropic media you may have more than one kind of transverse waves.
But it's not clear what you mean by "polarization". How do the rocks and minerals "polarise sound energy"? Do you have an example?
"Polarization" as an action (as opposite to a property: linear polarization for example) has to do with a change in the polarization state of the wave. Like when you have "unpolarized" light passing through a piece of polaroid and emerging as linearly polarized.
I suppose it's a matter of semantics and of the same word being used in several ways.

 

In case of a medium, "polarization" refers to the medium acquiring electric dipole momentum under influence of electric field - whether due to long-distance separation of charges (in conductors) or short distance induced electric moments.

The mechanical analogue of "polarization" is "strain". And the analogue of "polarizability" is "compressibility".

Note that elastic phenomena are more complex than electromagnetic. Elasticity tensor has what, 21 components? Even in completely isotropic environment, light has 1 speed, but sound has 2 (compression wave and shear wave speed).

 

You are implying it is interactions with individual atoms. When EM waves travel through a medium in a coherent way, it interacts with the bulk medium. If individual atoms were involved (as with the classic interaction between a photon and a Hydrogen atom), the phase of the wave would be disrupted because the re-emitted photons would not stay in phase with each other. As it is, the wave maintains its integrity as it travels through the medium.

 

The phase shifts would all be different because there is a statistical distribution associated with absorption and emission of a photon. That's the 'problem'; the wave front would no longer have integrity and you would no longer have a 'ray'. In order to explain what goes on you need to acknowledge that the photons are interacting with the whole thing. (If you really want to consider photons in this process which is essentially a wave phenomenon).

 

Polarisation has a rather specific meaning with waves and only applies to transverse waves (not P waves or sound). The 'polarisation' of molecules by an EM field is not the same thing and could better be termed displacement in this case.

 

But the wave DOES interact with individual atoms. And yes, it causes scattering.

Look at gases. The molecules in atmospheric or lower pressure gases are too far from another to allow light to interact with more than one molecule at a time, except on the rare occasions when molecules are undergoing collision.

The phases of light scattered from different air molecules are indeed out of phase.

Now, a wave of light encounters many air molecules over one wave. With the result that while the retarding/refracting effects of air molecules add up over many molecules, the scattered waves being out of phase undergo destructive interference. This is not complete because the positions of air molecules are random. There is Rayleigh scattering in air. But it is still relatively weak - much of the light can pass through long column of air and is left over from scattering, yet appreciably retarded.