Is the Momentum of a Photon Conserved in Matter?

• Apr 20, 2004

• #1

tozhan

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i have a question that'll really test your minds...or not...

Im still just getting to grips with some of this physics stuff but i can't work this out.

If light travels through an object it takes longer to pass through than in a vacuum right? now if heard this is because the photons are absorbed and reemitted by each particle of matter that they encounter. However this would mean that their momentum must somehow be conserved (the photons all carry on is the same direction until they leave the object).

How is this 'direction' conserved? my problem is that I've been taught that momentum is the product of mass times velocity. but if a photon has no mass it can't then have momentum! I am sure its explainable and that i just don't understand enough quantum mechanics yet to answer it myself.

If you think you can help, post away??

 

Last edited: Apr 21, 2004

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• Apr 21, 2004

• #2

selfAdjoint

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A photon of frequency [tex] \nu [/tex] has momentum [tex] h\nu [/tex] in the direction it is going, where h is Planck's constant. Einstein discovered this in the course of explaining the photoelectric effect, for which he got the Nobel prize.

It is not true that in advanced physics momentum is only defined by mass times velocity.

 

• Apr 21, 2004

• #3

Gokul43201

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Actually, p = hv / c. But in any case, the momentum of a photon passing through some material will not be conserved.

All materials absorb some fraction of the photons that pass through them, so these photons lose all their momentum (and energy, which goes into heating up the material). The fraction of photons absorbed depends on a property of the material called its absortion coefficient, A.

Then, some of the photons get reflected by the material - bouncing back is clearly a changing of direction - depending on it's reflection coefficient, R.

The rest of the photons (this fraction is proportional to the transmission coefficient, T=1-R-A) pass clean through the material, but even these photons have their monentum altered, because they are refracted by the material. They come out in a different direction than they go in. That's why light bends through water or glass, and the angle of deviation depends on the refractive index of the material, which is also defined as the ratio of the speed of light in vacuum to that in the material.

 

• Apr 21, 2004

• #4

turin

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I would add that the photons either maintain their momentum because they don't at all interact or they completely lose their identity once they interact. It is the momentum of the entire system (beam of photons + bulk of material) that is conserved, not necessarily the momentum of an individual photon, which may not even exist after the process.

 

• Sep 25, 2004

• #5

chifos

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what dose this mean dose a photon have momentum?

 

• Sep 25, 2004

• #6

danitaber

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chifos said:

what dose this mean dose a photon have momentum?

Chifos:
I refer you back to Self-Adjoint's post and Gokul's response directly below it.

A photon does, indeed, have momentum, although defined in a different way.

In dealing with conservation of "___blank___" issues, however, one looks at the whole system and does not zero in on a single item, which is just paraphrasing Turin's post.

Does this clarify it a bit, Chifos?

 

• Sep 26, 2004

• #7

reilly

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The idea of an electromagnetic momentum predates QM. That is, Poynting's Thrm of classical E&M shows that an electromagnetic field has both energy and momentum, which contribute to the total conserved energy and momentum. And, for the field, energy is directly proportional to momentum's magnitude, that is it is the same as that of a particle of zero mass.(In fact, you use Poynting's thrm to do just this.) Classical/symmetry/invariance considerations really force the photon's zero mass.
Regards,
Reilly Atkinson

 

• May 4, 2010

• #8

sah-sah

7

0

what is correlate the orbital angular momentum of photon with polarization EM wave?

reilly said:

The idea of an electromagnetic momentum predates QM. That is, Poynting's Thrm of classical E&M shows that an electromagnetic field has both energy and momentum, which contribute to the total conserved energy and momentum. And, for the field, energy is directly proportional to momentum's magnitude, that is it is the same as that of a particle of zero mass.(In fact, you use Poynting's thrm to do just this.) Classical/symmetry/invariance considerations really force the photon's zero mass.
Regards,
Reilly Atkinson

 

1. What is the momentum of a photon?

The momentum of a photon, also known as photon momentum, is the amount of motion that a photon possesses. It is a fundamental property of a photon and is related to its energy and wavelength.

2. How is the momentum of a photon calculated?

The momentum of a photon can be calculated using the equation p = E/c, where p is the momentum, E is the energy, and c is the speed of light. This equation is derived from Einstein's famous equation E=mc².

3. Does a photon have mass?

No, a photon does not have mass. It is a massless particle that only has energy and momentum. This is why it can travel at the speed of light, as objects with mass cannot reach this speed.

4. Can the momentum of a photon be changed?

Yes, the momentum of a photon can be changed by interacting with other particles or objects. When a photon is absorbed or emitted by an atom, its momentum changes. This is the basis for many technologies, such as solar panels and laser technology.

5. How does the momentum of a photon relate to the photoelectric effect?

The photoelectric effect is a phenomenon where photons can cause electrons to be emitted from a metal surface. The momentum of a photon is directly related to its energy and wavelength, which determines the amount of energy transferred to electrons during the photoelectric effect.