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Why photon doesn't change direction in medium?


DimaMazin
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Yes indeed a photon can change direction - it is called reflection (not refraction, that requires a change of medium sorry Strange).

But what it can't do is spontaneously change direction for no reason. That would contravene momentum conservation as John says.

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1 hour ago, Strange said:

It does (or can do). It is called refraction. 

It doesn't in one medium. Why entrance in atom  and exit are on one straight line which coincides with the direction?

Edited by DimaMazin
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58 minutes ago, DimaMazin said:

It doesn't in one medium. Why entrance in atom  and exit are on one straight line which coincides with the direction?

I think your question could be rephrased as: what determines the exit angle of a photon when it interacts with  an electron in an atom?

Edited by StringJunky
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1 hour ago, studiot said:

Yes indeed a photon can change direction - it is called reflection (not refraction, that requires a change of medium sorry Strange).

Surely both require a change in medium? But you are right, in that I assumed a change in medium because of the question. I think SJ has identified what question should be. 

1 hour ago, John Cuthber said:

That's scattering

Oh no! Here we go again. :)

 

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29 minutes ago, StringJunky said:

I think your question could be rephrased as: what determines the exit angle of a photon when it interacts with  an electron in an atom?

StringJunky,

I think that is a good rephrase.  And it brings up some additional questions in my mind.

When a photon "changes direction", is it the "same" photon, or has a quantum of energy boosted an electron to a higher energy level which then falls to a lower energy level releasing a "new" photon?  Same impulse of energy, but perhaps repackaged, in terms of it's direction of propagation?

How big is a photon's sphere of influence, as it is both particle and wave and is in essence an impulse disturbing electrical and magnetic fields in orthogonal fashion?

When a photon leaves an atom after being released by an electron falling to a lower energy level, does that wave of energy go out in all directions, like the ripples on a pond, only three dimensions, or is there a particular line, centered at the atom, on which the photon travels?

Regards, TAR

Edited by tar
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5 minutes ago, tar said:

StringJunky,

I think that is a good rephrase.  And it brings up some additional questions in my mind.

When a photon "changes direction", is it the "same" photon, or has a quantum of energy boosted an electron to a higher energy level which then falls to a lower energy level releasing a "new" photon?  Same impulse of energy, but perhaps repackaged, in terms of it's direction of propagation?

I've asked that question before and it's treated as the same photon, since how would you distinguish them? I kind of wonder the same thing and wonder if there is a change in momentum, dependent on the properties of the incident photon, which causes a predictable exit angle.

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23 minutes ago, Strange said:
1 hour ago, studiot said:

Yes indeed a photon can change direction - it is called reflection (not refraction, that requires a change of medium sorry Strange).

Surely both require a change in medium? But you are right, in that I assumed a change in medium because of the question. I think SJ has identified what question should be. 

No, reflection takes place entirely within one medium which the the photon never leaves.

Yes of course it occurs at a medium boundary.

Refraction does not occur until the photon has entered the second medium.

Of course the second medium may simple be a variant of the original, perhaps colder air or water.

13 minutes ago, tar said:

When a photon leaves an atom after being released by an electron falling to a lower energy level, does that wave of energy go out in all directions, like the ripples on a pond, only three dimensions, or is there a particular line, centered at the atom, on which the photon travels?

That's a bit like asking in a unverse that is empty apart from one single gas molecule, does that comprise a gas?

 

You don't get wavelike properties until you have a stream or repetition of (many) photons in the plural.

Edited by studiot
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14 minutes ago, studiot said:

You don't get wavelike properties until you have a stream or repetition of (many) photons in the plural.

Studiot,

So does one photon have a particular frequency and energy, or do you need plural to determine these attributes of light?

Regards, TAR

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1 hour ago, studiot said:

You don't get wavelike properties until you have a stream or repetition of (many) photons in the plural.

If I bounce a single photon off a ( well made, blazed) diffraction grating where is  it likely to go?

1 hour ago, StringJunky said:

I've asked that question before and it's treated as the same photon, since how would you distinguish them? I kind of wonder the same thing and wonder if there is a change in momentum, dependent on the properties of the incident photon, which causes a predictable exit angle.

Sometimes , it's clearly not the same photon, because it has changed colour.
https://en.wikipedia.org/wiki/Raman_scattering

 

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3 hours ago, DimaMazin said:

It doesn't in one medium. Why entrance in atom  and exit are on one straight line which coincides with the direction?

Why would it change direction? It's already been pointed out that momentum has to be conserved.

1 hour ago, tar said:

Studiot,

So does one photon have a particular frequency and energy, or do you need plural to determine these attributes of light?

Regards, TAR

Yes, it does. A single photon is absorbed in a transition, and it requires a particular frequency (or energy) in order to do so.  

Whether you need multiple photons to determine the frequency is an issue of how you are doing the measurement.

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4 minutes ago, StringJunky said:

What's happening when there's a change of direction if it's conserved? 

There's an interaction with the medium or something in the medium. e.g. refraction or reflection, or scattering. There's something else that is there which allows momentum to be conserved.

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4 minutes ago, swansont said:

Why would it change direction? It's already been pointed out that momentum has to be conserved.

Because photon delay has some time. For example gravitational  delay of photon  .  Also electrons are moving, the moving could make change during some time.

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Just now, swansont said:

There's an interaction with the medium or something in the medium. e.g. refraction or reflection, or scattering. There's something else that is there which allows momentum to be conserved.

You mean within the system it's conserved as a total? I was initially thinking the momentum was conserved within the photon and it has to exit with the same momentum. i think i need to read more of JC's suggestion into Raman scattering to figure out the difference.

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1 minute ago, StringJunky said:

You mean within the system it's conserved as a total? I was initially thinking the momentum was conserved within the photon and it has to exit with the same momentum. i think i need to read more of JC's suggestion into Raman scattering to figure out the difference.

A photon in a medium doesn't have the same as a photon in free space. There's a conundrum here, known as the Abraham-Minkowski controversy, because it's not clear if the photon's momentum gets bigger or smaller. Two theoretical approaches give different answers; the issue is that you can't ignore the medium, because light traveling though a medium is constantly interacting.

But if you had a prism, where the light entering and leaving happens at different angles, there would definitely be a force exerted on the prism, just like there's a force on a mirror from reflection.

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1 minute ago, swansont said:

A photon in a medium doesn't have the same as a photon in free space. There's a conundrum here, known as the Abraham-Minkowski controversy, because it's not clear if the photon's momentum gets bigger or smaller. Two theoretical approaches give different answers; the issue is that you can't ignore the medium, because light traveling though a medium is constantly interacting.

But if you had a prism, where the light entering and leaving happens at different angles, there would definitely be a force exerted on the prism, just like there's a force on a mirror from reflection.

Thanks. That at least clears it up insomuch as there is a change. Can that exchange cause a wavelength difference between the incident and exiting photon? Does that preserve the momentum of the photon; the difference being they have different energies?

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5 hours ago, DimaMazin said:

Why photon delay in atom of transparent medium doesn't change direction of photon?

There is no perfectly transparent medium. It's just a matter of how many meters, or kilometers, we're talking about.

If you point laser to water, you can see laser beam. It means that some photons from laser beam have been reflected from water molecules or contamination present in it.

 

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16 minutes ago, StringJunky said:

Thanks. That at least clears it up insomuch as there is a change. Can that exchange cause a wavelength difference between the incident and exiting photon? Does that preserve the momentum of the photon; the difference being they have different energies?

Inelastic scattering changes the energy of the photon. Photons of different energy must have different momentum.

34 minutes ago, DimaMazin said:

Because photon delay has some time. For example gravitational  delay of photon  .  Also electrons are moving, the moving could make change during some time.

There is no real interaction with any atoms (i.e. it's virtual, so there is no momentum transfer to the atoms.

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