refraction

[phil]

You know how refraction happens because light travels at different speeds in different materials?

What causes light to travel slower in other materials? I've been looking all over and haven't found an answer yet. Does anybody have an answer?

-phil

[Volker.Doormann]

Originally posted by phil
What causes light to travel slower in other materials? It is caused, if the material is more optical 'thick' than - for example vacuum. This optical 'thickness' is described by the complex dielectric function E (epsilon) of a material. E = E' + iE" , where E' is the real part and E" is the imaginary part of the dielectric function. E' = n^2 - k^2 and E" = 2nk, where n is the refractive index and k is the extinction coefficient describing the absorption of light. If a material is optical 'thick' for example diamond, this is related to a refractive index greater than 1.0 in the optical wavelength range of n ~2.4. This means that an optical light wavelength l (lambda), which is penetrating a diamond out of an other medium measure only the factor 1/2.4 as length in meters in the diamond, as it would measure in vacuum. Because the time, that a wave needs to move one wavelength distance in the diamond is equal to the time that a wave needs to move in vacuum, this reduction in the length l of the wavelength results in the imagination of a reduction of the light velocity, but this is not really true; you measure the optical path length then outside the diamond in a medium of n~1.

Volker

[markfiend]

Originally posted by phil
What causes light to travel slower in other materials?

To rephrase Volker's reply: There's stuff in the way.

[phil]

Volker,
Thanks for your reply. I know that the speed of light is determined by electrical permittivily and magnetic permeability and that diffferent substances have different values for these properties, but what is the mechanism that effects these properties? Is it the mass of the atoms or their electric and magnetic fields?

Everywhere I look there is only an explanation like gravity is explained. There is a formula that tells what is does but not how it does it. Perhaps it has yet to be explained.

Because the time, that a wave needs to move one wavelength distance in the diamond is equal to the time that a wave needs to move in vacuum, this reduction in the length l of the wavelength results in the imagination of a reduction of the light velocity, but this is not really true; you measure the optical path length then outside the diamond in a medium of n~1.
I can't seem to see how the speed reduction is imaginary. If the time is takes to traverse 1 wavelenth is the same and wavelenth is shortened, then there would be an actual speed reduction.

Say a car always travels between two lamposts along a road in the same amount of time. If the distance between lamposts decreases, then the car would have to slow down to make its time between the lamposts the same. This speed reduction is not imaginary.

-phil

[Corona688]

I've heard it involves the absorbtion and re-emission of photons..

[KeithHarwood]

Originally posted by Corona688
I've heard it involves the absorbtion and re-emission of photons..

One of Feynmann's books, I think `The Nature of Physical Law' has a good description of this.

[Volker.Doormann]

Originally posted by phil
Volker,
Thanks for your reply. I know that the speed of light is determined by electrical permittivily and magnetic permeability and that diffferent substances have different values for these properties, but what is the mechanism that effects these properties? Is it the mass of the atoms or their electric and magnetic fields? Mass not. Atoms are involved, but it is their outer electrons wich have an interaction with photons. Background of this is matter is an oscillator present in the material, that is able to transmit light (diamond, glass, ruby, single cristals, water, etc.), or many of them. This oscillator is able to oscillate in resonance with the same frequency of the light photons or with nearly the same frequency. Basic of this oscillator is the geometry in the crystal structure of the material. The distances of ions in the lattice can have lengths which can interact with photons comming across of the nearly the same wavelenght. This is the reason, why gold is reflecting a different light (spectrum) back, as for example diamond. The oscillator has an oscillator strength and a width. If the frequency of the photon is very different to the resonance frequency of an oscillator in the material, then the photon is transmitted with light velocity trough the material, because the permittivity is 1.0. If the frequency of the photon is equal to the resonance frequency of the oscillator, then the photon is absorbed in the oscillator, because of the high E" resonance value. It is the interaction of the photon with this oscillator, which can lead to a frequency minor or greater of the resonance frequency, where the photon perceives an enhancement of the natural permittivity of 1.0 through this oscillator without a great loss in intensity from extinction (E"). The relation of the velocity of the photons is coupled to this. If the permittivity is lower then 1.0, then the lightvelocity of the photon is greater as in vacuum, and this is also coupled to a refractive index of lower then 1.0. Is the natural permittivity of 1.0 in vacuum enhanced by the dielectric character of the oscillator in the material, then the velocity of the photon is reduced according to the refractive index - in comparison to the vacuum or air, where we measure mostly the velocity of the light photons. Maybe an imagination helps, in that the photon is not only moving linear in length, but in circle paths of single wavelengths. The photon has an amplitude with two polarities, and is interacting with the permittivity. On each circle of moving, the photon moves a length of one wavelength, and in the case of resonance of the photon with the oscillator of the material, this movement in length is reduced, because the nature of the permittivity is changed to a more concentrated permittivity in the oscillator. If there would no permttivity in a medium, for example E' = 0.0 , then the light velocity in that medium would be infinite. It is a very natural relation between the light velocity and the permittivity and the refractive index. In the case of x-ray photons the frequency is in a range of an oscillator of material (metal), where the refractive index is lower the 1.00, and the velocity is greater as the light velocity in vacuum. The length of a light wave in meters is defined by the frequency and the permittivity. If the permittivitty is changing because of any circumstance (oscillator function f(frequency)), then this is related strongly to a change in the velocity of the light photons.
I can't seem to see how the speed reduction is imaginary. If the time is takes to traverse 1 wavelenth is the same and wavelenth is shortened, then there would be an actual speed reduction.
Correct. Measuring the length in vacuum outside the material, and calculate the velocity, this in a real decrease in velocity. It is only the time needed (frequency) of one length of a wave in each medium which is constant, because the reduction in length for a reduced velocity doesn't change it's relation.

Volker

[phil]

Thanks peoples!:)
It's amazing stuff like that isn't explained in more detail in physics books. I guess I just haven't found the right books yet. I have an old physics textbook from the 80's that my dad has used. I'll be getting a new one for my use in physics class. Perhaps it will have some more up to date info. Thanks again.

-phil

[Volker.Doormann]

Originally posted by phil
It's amazing stuff like that isn't explained in more detail in physics books. I guess I just haven't found the right books yet. I have an old physics textbook from the 80's that my dad has used. I'll be getting a new one for my use in physics class. Perhaps it will have some more up to date info. [/B] If you interested in more deep understanding of solid state physics, and you are willing to by a book, you can take a view on: Introduction to Solid State Physics by Charles Kittel (http://www.amazon.com/exec/obidos/tg/detail/-/0471111813/qid=1061371263/sr=1-1/ref=sr_1_1/002-6575465-6004801?v=glance&s=books). AFAIK classical basics of solid state physics are not out of date, so alternatively you can make us of an used older version.

Volker