# The Naked Scientists Forum

### Author Topic: In what way is a reflected photon degraded?  (Read 22649 times)

#### simplified

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##### In what way is a reflected photon degraded?
« Reply #25 on: 09/11/2011 14:58:41 »
That's a tricky one. In my book it loses 'energy' by losing momentum. To assume otherwise is to define momentum as something else than 'energy'. But momentum will transfer a 'energy' if we define it as the ability to do work, and transform. So, I don't think you can do that, differ them.

I can't, but ability to impart momentum is not ability to impart energy.

#### JP

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##### In what way is a reflected photon degraded?
« Reply #26 on: 09/11/2011 15:29:45 »
Energy and momentum are not the same thing by any definition.  Momentum is given by three numbers and energy by one number for starters.

Now, under some conditions, you can use conservation of energy/momentum/mass to figure out one from the other, but that doesn't mean they're the same thing.

#### yor_on

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##### In what way is a reflected photon degraded?
« Reply #27 on: 09/11/2011 15:39:51 »
Well, I'm saying that momentum becomes 'energy' in its transformation/interaction. Am I wrong?
=

Then again, thinking of a 'light wave' and trying to see its momentum becomes really tricky as one prefer to define a momentum to something specific, like a 'photon/particle'. So how do quantum mechanics find the momentum for a standing wave? Its 'energy' alone. And how about a wave existing 'everywhere'? Where is the momentum?

It's a weird subject :)
« Last Edit: 09/11/2011 15:51:42 by yor_on »

#### JP

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##### In what way is a reflected photon degraded?
« Reply #28 on: 09/11/2011 15:50:01 »
What do you mean by its transformation?  Isn't that assuming it's becoming something that isn't momentum?  It should always stay momentum, and momentum in = momentum out.

#### yor_on

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##### In what way is a reflected photon degraded?
« Reply #29 on: 09/11/2011 15:53:47 »
Well, if you want to define it as something not being 'energy', then it has to transform into 'energy' when interacting. As long as I'm thinking right here that is. But I agree, that wasn't the best formulation, interaction is clearer.
=

What I mean is that 'momentum' is an expression of energy, and I find it very hard to see it any other way. But then we have interactions, in where the energy expressed will belong to to both particles interacting. When light blue shift/red shift it is a consequence of interactions, that is two 'frames of reference' 'communicating', as in that light being annihilated on your retina, not an expression of a 'photon' changing energy intrinsically.
« Last Edit: 09/11/2011 16:09:14 by yor_on »

#### yor_on

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##### In what way is a reflected photon degraded?
« Reply #30 on: 09/11/2011 15:59:59 »
Would you expect the momentum to stay the same (for the wave)? Isn't the conservation law implied for the whole 'system', not the reflected wave. That is, the momentum, counting in both mirror and wave, is unchanged?

Or is it something else you mean?

#### simplified

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##### In what way is a reflected photon degraded?
« Reply #31 on: 09/11/2011 16:30:49 »
Would you expect the momentum to stay the same (for the wave)? Isn't the conservation law implied for the whole 'system', not the reflected wave. That is, the momentum, counting in both mirror and wave, is unchanged?

Or is it something else you mean?
was p + 0
then -p + 2p
« Last Edit: 09/11/2011 16:40:04 by simplified »

#### JP

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##### In what way is a reflected photon degraded?
« Reply #32 on: 09/11/2011 16:31:47 »
Momentum of the whole system stays the same in the interaction.
Energy of the whole system stays the same in the interaction.

That's why you can't transform one to the other.  If momentum somehow transformed into energy, you'd violate conservation of both energy (you'd gain some) and momentum (you'd lose some).  What can happen is that different parts of the system gain or lose energy/momentum, but the total in the system has to stay constant for both energy and momentum, independently.

#### yor_on

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##### In what way is a reflected photon degraded?
« Reply #33 on: 09/11/2011 16:49:22 »
Yes, that's how I understands it. Transform here is my way of saying what you define as, gain or lose 'energy'. But it's the interaction creating it, not some magical, each one by themselves, 'happening' :)

Didn't know you couldn't use the word transform for this btw. Would it be wrong to say that radiation transforms into energy too?
=

Perhaps convert would be more acceptable :)
« Last Edit: 09/11/2011 16:52:53 by yor_on »

#### JP

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##### In what way is a reflected photon degraded?
« Reply #34 on: 09/11/2011 18:20:51 »
Well, like everything else in science, precision is king.  If you're precise about what you mean by transform, it's fine to use it in a scientific context.

But yes, in interactions particles can transfer energy or momentum.  It's possible for this energy and momentum to create new particles as well.

#### JP

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##### In what way is a reflected photon degraded?
« Reply #35 on: 09/11/2011 23:53:52 »
Also, it's worth checking out these three pages on wikipedia.  They go into details about how elastic scattering of light from an electron (i.e. it doesn't lose energy) is just a limit of the more general scattering where the energy loss is so small that it can be neglected:
http://en.wikipedia.org/wiki/Thomson_scattering
http://en.wikipedia.org/wiki/Compton_scattering
http://en.wikipedia.org/wiki/Klein-Nishina_formula

The last link actually provides the quantum model that extends from the elastic limit to the inelastic limit.

#### MikeS

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##### In what way is a reflected photon degraded?
« Reply #36 on: 11/11/2011 08:57:41 »
Thinking about it, presumably a photon can not be reflected.  To be reflected there would have to be a point at which it was stopped.  If not reflected then it must be absorbed and another photon re-emitted.

There seems to be two trains of thought on this.
One, the photon is re-emitted at the same frequency, in which case it cannot have imparted any momentum to the mirror.
Two, the photon imparts momentum to the mirror and another photons is re-emitted at a lower frequency.
Either way the 'reflected' photon does not impart any momentum to the mirror as there is no reflection.  The photons impart momentum on absorption and again on re-emission.  The more reflective the surface, the higher the frequency of the re-emitted photon.

If the above is correct then solar sails do not work by reflecting light but by absorbing and re-radiating it.
What's wrong with this argument?

#### yor_on

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##### In what way is a reflected photon degraded?
« Reply #37 on: 11/11/2011 09:14:12 »
What hurts me head here is the assumption that a interaction can leave what interacts untouched. In this case 'photons' bouncing of a mirror. So maybe you can differ a photons momentum from 'energy', although I don't see how. No, on second thought, I don't think you can. To use the idea of them 'bouncing' and keeping their energy, and then say it is proved by the way photons don't redshift ignore the fact that they are constantly sent out from a source, get annihilated at a sink, and re-emitted depending on their 'energy' relative what they annihilate against. What is even more irritating is 'proving' it using wave functions. It's rather naive to define a duality from one point of view solely, as long as there is no experimental proof of one side of it being secondary and dependent on the first. I won't do that.

I still think of it as a duality, not a wave packet.

#### yor_on

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##### In what way is a reflected photon degraded?
« Reply #38 on: 11/11/2011 09:24:42 »
And a photon doesn't have a frequency, only a momentum and a energy. It is waves that have a frequency, and a wavelength. Ah, that also mean that a individual 'photon' can't red shift, only 'waves' do that. And even if considering them over a extended period of time passing some 'point', the photons passing when measured have not red shifted. There must be some underlying definition for how they express the duality, in different situations, but I don't think considering them as 'renormalized wavepackets' with arbitrarily made 'cutoffs' answer that one. There is something else governing the way they express themselves. There is something inherently wrong with any approach that need to lift out the 'wrong' numbers, to get to the right ones, we see experimentally.
« Last Edit: 11/11/2011 09:34:18 by yor_on »

#### JP

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##### In what way is a reflected photon degraded?
« Reply #39 on: 11/11/2011 14:27:26 »
Yor_on, photons do have a frequency.  They don't oscillate like classical waves, but they have a property called wavelength.  This is necessary, because when you add them up in the right way, this property is what creates a classical wave with a given wavelength.  That frequency, f, is also what gives them energy of hf.

#### JP

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##### In what way is a reflected photon degraded?
« Reply #40 on: 11/11/2011 14:29:27 »
Mike, your reasoning is correct.  Reflection, absorption/re-emission and scattering from a mirror are all different words for the same phenomenon.  You can't reflect without giving up energy, but the usual assumption is that the energy loss is so small that it doesn't matter.

#### yor_on

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##### In what way is a reflected photon degraded?
« Reply #41 on: 11/11/2011 20:29:32 »
I don't know if I agree, I think of it as a duality? Maybe you are thinking of trapping photons in microwave cavities? How about Compton scattering, describing the opposite?

" The Compton effect is introduced in Gasiorowicz (in section 1.3, pages 7-9 in the 3rd edition, or pages 11-13 in the 2nd edition). The text describes the experimental discovery of the effect discovered by Arthur H. Compton - radiation of a given wavelength (in X-rays) sent through a foil was scattered in a manner inconsistent with classical radiation theory.

If one is dealing with elastic scattering,the system can be understood quantitatively as Thomson scattering. However, the Compton effect can be understood as photons scattering inelastically off individual electrons."

Here you start with x-rays, and end with 'photons'. Photoelectrons, Compton and Inverse Compton Scattering
« Last Edit: 11/11/2011 20:34:53 by yor_on »

#### JP

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##### In what way is a reflected photon degraded?
« Reply #42 on: 11/11/2011 21:03:25 »
You don't agree with what?

Photons have a frequency, which is true: E=hf after all.  If you want to know how this relates to the frequency of a classical wave, check out coherent states.  It takes a lot of math to prove it, but you can superimpose photons in such a way to make a classical wave of the same frequency as the photons: http://en.wikipedia.org/wiki/Coherent_states

Also, check my links above if you don't believe me that Compton and Thompson scattering are related.  The general quantum scattering problem is quite complex, but you can simplify it by assuming certain things, which is what is done in these two cases.  Thompspon scattering assumes that the photon loses so little energy that you can treat it as losing none and still get accurate results.
« Last Edit: 11/11/2011 21:07:39 by JP »

#### yor_on

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##### In what way is a reflected photon degraded?
« Reply #43 on: 11/11/2011 22:34:04 »
I don't agree to it being one or the other, to me radiation seems to define itself through the relations/interactions with what surrounds it in the measurement.

#### yor_on

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##### In what way is a reflected photon degraded?
« Reply #44 on: 11/11/2011 22:36:36 »
And no JP, they are related, the link I gave says it too.

#### JP

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##### In what way is a reflected photon degraded?
« Reply #45 on: 11/11/2011 23:58:26 »
So you're agreeing with me?

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##### In what way is a reflected photon degraded?
« Reply #46 on: 12/11/2011 04:26:39 »
Quote
A reflected photon imparts pressure to the reflecting surface.
This would imply that work has been done.
The photon has not been absorbed but it has been degraded by the loss of energy in some form.
In what way is the photon degraded by the loss of energy?

From what I have noticed throughout life is that whenever a force is exerted and deflected it does lose energy in some way.  Just as when light is deflected when bent through space as it is gravitationally bent as well considering how far stars are away from us.  A case in point would be the stars that we see at night and 'appear' to be in a fixed position in the sky.  Considering how far they are away, they may be a bit off from where they appear.  The closer the star of course the less it may be off, such as Alpha centauri the closest one to us if I recollect right.  The image is accurate of course, it just may not be exactly where we see it in space.  As far as my point goes, concerning loss of energy, have you ever known anything in life that that has a 'theoretical' constant velocity that didn't lose some type of momentum or energy when acted upon by some external force that is fixed?  One has to apply this formula when taking this aspect into consideration and make the assumption that this is equated in some way.

#### yor_on

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##### In what way is a reflected photon degraded?
« Reply #47 on: 12/11/2011 13:41:51 »
I don't know how you see it? Most people today define it from waves, with photons becoming some sort of 'focus' for that. I don't think either one is perfect, on the other hand I can't say how it should work. My own, most unscientific opinion, is that both waves and 'photons' are a description of what relations there are defining them.

That doesn't necessarily demand a underlying 'reality' aka, some 'hidden' background, defining what they really, really, are. But I think it demand another way of looking at them, that make their duality a result of the demands, different in different experiments. So I'm happy with thinking of both as 'real', well, as real as we can define it. Which then means that I'm not convinced of anything :)

#### JP

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##### In what way is a reflected photon degraded?
« Reply #48 on: 12/11/2011 14:15:07 »
Ah, I think I understand you now.  Check out that link I posted above (http://en.wikipedia.org/wiki/Coherent_states).  It explains the precise mathematical connection between photons and waves.

#### yor_on

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##### In what way is a reflected photon degraded?
« Reply #49 on: 13/11/2011 14:51:50 »
Yep JP. it's better to discuss it from coherent states, that precludes any of our ideas on 'waves and 'photons'' But then I meet another problem. I do like indeterminacy (HUP), but I'm not sure I find 'virtual particles' as convincing. To me they are starting to become a figment of our deep conviction of thingies 'moving and propagating'. And I'm not sure what that is any more. Physics are truly confusing, reminding me of Pandoras box. You may open it, but on your own peril. There has to be a way of describing it not involving 'virtuality' as in something 'moving'.

And to me it must have to do with 'time'.

I prefer indeterminacy to 'virtual particles' those days
« Last Edit: 13/11/2011 14:54:35 by yor_on »

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##### In what way is a reflected photon degraded?
« Reply #49 on: 13/11/2011 14:51:50 »