# What causes light to speed up after passing through something

Before i begin i would just like to say I am only an AS level student so my understanding of this area of physics may be quite limited but i am just interrested to know.

If light is travelling through a vacum at speed A and comes across a pane of glass, it will slow down to speed B, either instantly or due to decelaration, not sure which, after leaving the pane of glass it will then speed back up to speed A, again either instantley or due to accelaration.

My question is, where does the energy come from that causes the light to speed up again after leaving the pane of galss or wheather it even slows down atall, but instead it amplitude or wavelength either increases or decrease.

Any information on this subject would be much appreaciated as ive been trying to get my head around for a long time and have hit a mental block.

Janesh

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sophiecentaur
Gold Member
Hi
When you say that you need energy to speed something up, you are assuming that it has Mass (i.e. its Kinetic Energy would increase). Light doesn't have Mass so that isn't a problem when it emerges from glass into air and speeds up. Its Energy (the energy of the photons) is defined by its frequency, which remains the same across the interface.

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Drakkith
Staff Emeritus
The key is that light is an electromagnetic wave. When a wave, of any type, passes through a medium that is of a different refractive index it will change speeds. This does not require energy.

So the lights energy remains constant throughout the whole process but the resistivity of the median it is travelling through increases causing teh decelaration, but when the median reduces in resistivity, because the energy has remained the same it goes back to orginal speed?

Drakkith
Staff Emeritus
So the lights energy remains constant throughout the whole process but the resistivity of the median it is travelling through increases causing teh decelaration, but when the median reduces in resistivity, because the energy has remained the same it goes back to orginal speed?
Yes, the light keeps the same amount of energy as it passes through. Light doesn't experience any acceleration or deceleration however. Once it enters or leaves a material it simply travels at the new speed instantly.

sophiecentaur
Gold Member
So the lights energy remains constant throughout the whole process but the resistivity of the median it is travelling through increases causing teh decelaration, but when the median reduces in resistivity, because the energy has remained the same it goes back to orginal speed?
Resistivity is not the relevant factor. If there is a resistive (loss) element in the medium, then some of the light energy will be absorbed but the frequency of the light passing through is not changed. You are still making 'mechanical' assumptions in your mental picture of what is happening, I think.

DrGreg
Gold Member

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davenn
Gold Member
2019 Award
This is really so frustrating. comments in that FAQ go directly against what Fyenman taught. And one is left in the middle saying who the heck do I believe ????!!

Is Fyenman wrong ? Have the theories changed dramatically since his time ? Was his brilliance just BS ?

Dave

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Bobbywhy
Gold Member
This is really so frustrating. comments in that FAQ go directly against what Fyenman taught. And one is left in the middle saying who the heck do I believe ????!!

Is Fyenman wrong ? Have the theories changed dramatically since his time ? Was his brilliance just BS ?

Dave
Dave, I read the FAQ. Will you please say what are the discrepancies between what Feynman taught and the FAQ.

Regards, Bobbywhy

davenn
Gold Member
2019 Award
Dave, I read the FAQ. Will you please say what are the discrepancies between what Feynman taught and the FAQ.

Regards, Bobbywhy
well I can but is there really any point ??

I find that no one on this board seems to follow anything Fyenman said anyway specially when it comes to describing light as particles and their interactions with media as he does.
hence my further comment .... Have the theories changed dramatically since his time ?

Have they really changed ? do I need to forget about the things he taught ?

from the FAQ here was the one that hit me only a few sentences into it....

A common explanation that has been provided is that a photon moving through the material still moves at the speed of c, but when it encounters the atom of the material, it is absorbed by the atom via an atomic transition. After a very slight delay, a photon is then re-emitted. This explanation is incorrect and inconsistent with empirical observations.
lets get a clarification.... which part of the explanation is incorrect ?

1) but when it encounters the atom of the material, it is absorbed by the atom via an atomic transition
2) After a very slight delay,
3) a photon is then re-emitted.

or the whole statement ?

Dave

sophiecentaur
Gold Member
The expression "via an atomic transition" may be taken wrongly because the dreaded H atom looms, with its discrete levels. In a condensed state, there isn't a series of levels but a continuum. You cannot just consider an atom on its own. I don't think it makes any difference whether you consider interaction with an atom with modified levels (a continuum) or with the whole distributed charge system. There is no conflict, is there?

davenn
Gold Member
2019 Award
The expression "via an atomic transition" may be taken wrongly because the dreaded H atom looms, with its discrete levels. In a condensed state, there isn't a series of levels but a continuum. You cannot just consider an atom on its own. I don't think it makes any difference whether you consider interaction with an atom with modified levels (a continuum) or with the whole distributed charge system. There is no conflict, is there?
well there is if you take the statement as a whole, which is why I asked for clarification as to what part of the statement is deemed incorrect and inconsistent with empirical observations.

Fyenman plainly stated in one of his lectures that photons were absorbed and new photons were emitted. (at this time I dont care about the "slight delay" part of the statement) I just want to know if the " incorrect and inconsistent with empirical observations. " was referring to the statement as a whole or an individual part of it.
If referring to the statement as a whole then it goes against what Fyenman said

Dave

PS I am no expert and am alway willing to learn new things, which again is why I asked " has theory changed since Fyenman ? have we learnt new ways to describe processes ?

the above is just 1 example of times when reading stuff in various threads on PF that I have thought to myself... "now thats NOT what Fyenman taught" and it just leaves me wondering who to believe ??

Bobbywhy
Gold Member
"The speed at which light propagates through transparent materials, such as glass or air, is less than c. The ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = c / v). For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 ≈ 200,000 km/s; the refractive index of air for visible light is about 1.0003, so the speed of light in air is about 90 km/s slower than c."
http://en.wikipedia.org/wiki/Speed_of_light#In_a_medium

"At the microscale, an electromagnetic wave's phase speed is slowed in a material because the electric field creates a disturbance in the charges of each atom (primarily the electrons) proportional to the electric susceptibility of the medium. (Similarly, the magnetic field creates a disturbance proportional to the magnetic susceptibility.) As the electromagnetic fields oscillate in the wave, the charges in the material will be "shaken" back and forth at the same frequency. The charges thus radiate their own electromagnetic wave that is at the same frequency, but usually with a phase delay, as the charges may move out of phase with the force driving them (see sinusoidally driven harmonic oscillator). The light wave traveling in the medium is the macroscopic superposition (sum) of all such contributions in the material: The original wave plus the waves radiated by all the moving charges. This wave is typically a wave with the same frequency but shorter wavelength than the original, leading to a slowing of the wave's phase speed. Most of the radiation from oscillating material charges will modify the incoming wave, changing its velocity. However, some net energy will be radiated in other directions or even at other frequencies (see scattering).

Depending on the relative phase of the original driving wave and the waves radiated by the charge motion, there are several possibilities:

• If the electrons emit a light wave which is 90° out of phase with the light wave shaking them, it will cause the total light wave to travel more slowly. This is the normal refraction of transparent materials like glass or water, and corresponds to a refractive index which is real and greater than 1.

• If the electrons emit a light wave which is 270° out of phase with the light wave shaking them, it will cause the total light wave to travel more quickly. This is called "anomalous refraction", and is observed close to absorption lines, with X-rays, and in some microwave systems. It corresponds to a refractive index less than 1. (Even though the phase velocity of light is greater than the speed of light in vacuum c, the signal velocity is not, as discussed above). If the response is sufficiently strong and out-of-phase, the result is negative refractive index discussed below.

• If the electrons emit a light wave which is 180° out of phase with the light wave shaking them, it will destructively interfere with the original light to reduce the total light intensity. This is light absorption in opaque materials and corresponds to an imaginary refractive index.

• If the electrons emit a light wave which is in phase with the light wave shaking them, it will amplify the light wave. This is rare, but occurs in lasers due to stimulated emission. It corresponds to an imaginary index of refraction, with the opposite sign as absorption.

For most materials at visible-light frequencies, the phase is somewhere between 90° and 180°, corresponding to a combination of both refraction and absorption."
http://en.wikipedia.org/wiki/Refractive_index

I guess the reason light "speeds up" when leaving the medium is just that the refractive index of air is less that the glass.

sophiecentaur
Gold Member
well there is if you take the statement as a whole, which is why I asked for clarification as to what part of the statement is deemed incorrect and inconsistent with empirical observations.

Fyenman plainly stated in one of his lectures that photons were absorbed and new photons were emitted. (at this time I dont care about the "slight delay" part of the statement) I just want to know if the " incorrect and inconsistent with empirical observations. " was referring to the statement as a whole or an individual part of it.
If referring to the statement as a whole then it goes against what Fyenman said

Dave

PS I am no expert and am alway willing to learn new things, which again is why I asked " has theory changed since Fyenman ? have we learnt new ways to describe processes ?

the above is just 1 example of times when reading stuff in various threads on PF that I have thought to myself... "now thats NOT what Fyenman taught" and it just leaves me wondering who to believe ??
That statement can be taken in two ways, I think. A FAQ is not 'Gospel' and only receives a certain amount of Editorial Attention. It may need some 'interpretation' - which is fair enough because both the Bible and Koran need it too!
The empirical evidence that exists is that materials exhibit broad band behaviour. That merely implies that the interaction with single atoms is not the same as it would be with an isolated atom. It seems to me that the FAQ is knocking on the head the simplistic model of a solid consisting of individual and isolated atoms, which would have discrete energy states. If you adjust your model to include the real behaviour of atoms in a solid then there is no problem - is there?

ZapperZ
Staff Emeritus
People need to understand that the FAQ was meant to give a naive explanation of light transport IN A SOLID. Light transport in gasses, anomalous medium, etc.. etc. can have a different explanation. However, a lot of the questions being asked on this forum have been about light moving in glass, etc.. etc.. i.e. SOLIDS.

In solids, as has been stated, you no longer have ISOLATED ATOMS. Here, the collective behavior of the material can have a DOMINANT effect. Phonons do NOT exists in isolated atoms, but they do exist in solids, so much so that the properties of phonons often dictate many of the properties of the solid that we are familiar with! Light does not get 'absorbed and retransported' via discrete spectrum. Yet, we see BROAD BAND, continuous spectrum being transmitted. This clearly shows that individual atoms are not responsible for such transport through the material!

Feynman was not a condensed matter physicist. In fact, he could not solve superconductivity - it took BCS to do that.

Zz.

sophiecentaur
Gold Member
People need to understand that the FAQ was meant to give a naive explanation of light transport IN A SOLID. Light transport in gasses, anomalous medium, etc.. etc. can have a different explanation. However, a lot of the questions being asked on this forum have been about light moving in glass, etc.. etc.. i.e. SOLIDS.

In solids, as has been stated, you no longer have ISOLATED ATOMS. Here, the collective behavior of the material can have a DOMINANT effect. Phonons do NOT exists in isolated atoms, but they do exist in solids, so much so that the properties of phonons often dictate many of the properties of the solid that we are familiar with! Light does not get 'absorbed and retransported' via discrete spectrum. Yet, we see BROAD BAND, continuous spectrum being transmitted. This clearly shows that individual atoms are not responsible for such transport through the material!

Feynman was not a condensed matter physicist. In fact, he could not solve superconductivity - it took BCS to do that.

Zz.
OH boy, you'll have comments about that. Did you not know that he walked on water and that there have been no advances since him?

ZapperZ
Staff Emeritus
OH boy, you'll have comments about that. Did you not know that he walked on water and that there have been no advances since him?
They can bring it on! I have history on my side. And all those people have are idol worship.

http://arxiv.org/abs/1008.0447

Zz.

sophiecentaur