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Offline MikeS

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In what way is a reflected photon degraded?
« on: 31/10/2011 08:16:19 »
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?


 

Offline Soul Surfer

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In what way is a reflected photon degraded?
« Reply #1 on: 31/10/2011 12:07:29 »
Work is done only if the surface actually moves. In most cases of most surfaces the collisions are perfectly elastic the surface does not move as a whole and so no energy is lost.  The same would be true for grains of dust etc. Even when photons of light are reflected or scattered by individual atoms the atoms are much heavier than the momentum of the photons and scattering is usually elastic see Rayleigh Scattering  however this may not be the case particularly for higher energy photons and energy will be lost  see Compton scattering.

An interesting aside.  In general for higher energy photons like gamma rays the reaction of the emitting nucleus causes a spreading of the wavelength  however there are some cases where the nucleus is effectively locked into the structure by quantum effects and the energy of the photon is incredibly precise and this can be used for very accurate measurements foe example the gravitational red shift caused by the earth's gravity.  this is called The Mossbauer effect.
 

Offline simplified

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In what way is a reflected photon degraded?
« Reply #2 on: 31/10/2011 16:44:17 »
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?
In your case:
E -lost energy
m' - zero mass of photon = hν/c
m - mass of surface
c -  speed of light
                         E=2(m')c/m
 

Offline Bored chemist

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In what way is a reflected photon degraded?
« Reply #3 on: 31/10/2011 21:35:21 »
The photons can lose energy when they bounce off things, but the effect is usually only noticeable when the things are small.
http://en.wikipedia.org/wiki/Compton_scattering
 

Offline MikeS

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In what way is a reflected photon degraded?
« Reply #4 on: 02/11/2011 07:11:27 »
Thanks guys but I don't think your replies really answers the question "In what way is the photon degraded by the loss of energy?"

A photon can not loose energy by slowing down and as I understand it, it is not red-shifted.  So what is happening?  Could it be that some photons are being reflected with no energy transfer and some are being absorbed?
 

Offline Soul Surfer

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In what way is a reflected photon degraded?
« Reply #5 on: 02/11/2011 15:38:38 »
if a photon loses energy it is red shifted  or more accurately absorbed and remitted with a lower energy with the result that the particle it hit moves off with the energy deficit in a direction associated with the absorbed and emitted photons
 

Online yor_on

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In what way is a reflected photon degraded?
« Reply #6 on: 02/11/2011 18:32:36 »
I'm not sure I get this one? Are you suggesting that photons don't get absorbed by a mirror but instead are 'reflected'? There are two ways to describe it, and using a wave picture is the oldest one and also the one we easiest recognize. The other is a particle picture in where the photons get absorbed by the mirrors atoms, most releasing new photons, that over a broad distribution presents us with the angles of reflection we associate with a wave. Although some of the explanations to how a 'photon' is expected to do so seem to be connected to the the way a wave interfere, quenching and reinforcing, via 'frequencies' which, as far as I know, no 'photon' is expected to have in a particle picture, frequency that is. Another way to define their angles 'reflecting' might be the momentum they have and how that gets 'mirrored' by the atom creating new photons.
 

Offline Soul Surfer

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In what way is a reflected photon degraded?
« Reply #7 on: 03/11/2011 08:58:43 »
I was referring to MikeS's statement a photon is not red shifted when it is involved in nonelastic scattering.  You seem to have a hangup about the wave/particle duality it is a true duality photons always have the full properties of both. Can you not think of a wave packet?   true as the photon energy rises it is easier to think of them as particles.  Processes inside the limits of quantum uncertainty cannot be observed it is only the results that matter
 

Offline MikeS

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In what way is a reflected photon degraded?
« Reply #8 on: 04/11/2011 09:08:42 »
If it's a perfectly reflecting surface, presumably there would be no loss.  The photon being absorbed and re-emitted at the same frequency and the surface recoiling in the process.  This would imply a gain of energy, so it can't happen.  But what is happening?  Is the photon red-shifted by being reflected by a perfectly reflecting surface?  Can a photon be absorbed and re-emitted by a perfectly reflecting surface?  If the surface is perfectly reflecting how can it absorb the photon?  If it's a perfectly reflecting surface does the photon impart any energy to that surface?
Thanks

Soul Surfer
Having read your previous replies I can't see how Rayleigh Scattering could be the answer as it applies to transparent objects.
« Last Edit: 04/11/2011 12:44:08 by MikeS »
 

Offline Soul Surfer

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In what way is a reflected photon degraded?
« Reply #9 on: 05/11/2011 00:20:24 »
Consider a largely but not perfectly reflecting heavy surface.  If a photon strikes the surface it can be either reflected or absorbed by the surface.  A photon has both energy and momentum if it is absorbed the energy in the photon warms the surface which could cause another photon of lower energy to be emitted.  If it is reflected the photon is unchanged except that its direction of travel has changed and therefore its momentum has changed so the law of conservation of momentum applies and some momentum has been given to the surface.  Note the momentum of a photon is very much less than its energy (a factor of the speed of light).  An illustration of this process is given in the behaviour of the Crookes radiometer.  as the pressure in the tube is reduced from lowish to a hard vacuum.

Why do you think that Rayleigh scattering only applies to "transparent " objects?
 

Offline MikeS

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In what way is a reflected photon degraded?
« Reply #10 on: 05/11/2011 08:41:51 »
Consider a largely but not perfectly reflecting heavy surface.  If a photon strikes the surface it can be either reflected or absorbed by the surface.  A photon has both energy and momentum if it is absorbed the energy in the photon warms the surface which could cause another photon of lower energy to be emitted.  If it is reflected the photon is unchanged except that its direction of travel has changed and therefore its momentum has changed so the law of conservation of momentum applies and some momentum has been given to the surface.  Note the momentum of a photon is very much less than its energy (a factor of the speed of light).  An illustration of this process is given in the behaviour of the Crookes radiometer.  as the pressure in the tube is reduced from lowish to a hard vacuum.

Why do you think that Rayleigh scattering only applies to "transparent " objects?

Soul Sufer
Thanks for reply.  I had been trying to find information on the Crooks radiometer but couldn't as I didn't know what it was called.  Interesting, I always thought it operated directly by radiation pressure but apparently its a heat engine.

I didn't know anything about Rayleigh scattering so did a quick internet search and it seemed to only be happening in transparent mediums like gasses.

"If it is reflected the photon is unchanged except that its direction of travel has changed and therefore its momentum has changed so the law of conservation of momentum applies and some momentum has been given to the surface." 
But "if some momentum has been given to the surface" then surely, the photon can not be "unchanged". It would have to be re-emitted at a lower frequency in order to conserve energy. Or, if it were reflected unchanged, then it can't have transferred any energy to the reflecting surface. ??????

We know that the reflected photon does not loose energy (red-shift).  So does it impart momentum to the reflecting surface?  Or is it only absorbed photons that impart momentum?
« Last Edit: 05/11/2011 09:01:25 by MikeS »
 

Offline Soul Surfer

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In what way is a reflected photon degraded?
« Reply #11 on: 05/11/2011 18:09:32 »
Even individual atoms are too heavy to allow a normal light photon to loose a significant amount of its total energy and momentum
 

Offline MikeS

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In what way is a reflected photon degraded?
« Reply #12 on: 06/11/2011 10:48:40 »
Even individual atoms are too heavy to allow a normal light photon to loose a significant amount of its total energy and momentum

But my point is any transference of energy and momentum, no matter how small must show as a change in the photons energy for the law of conservation of energy to hold.

This is a quote about solar sails.
"How does light push a solar sail?
Photons, which are "particles" of light, bounce off the reflective material of the sail. (Newton's Third Law of Motion states that for every action there is an equal and opposite reaction.) The reaction here causes a change in momentum, pushing the sail and accelerating the spacecraft. A photon reflecting off the mirror-like surface of a solar sail gives the sail a double kick -- a push equal to twice the photon's momentum (one push from the sail stopping the photon and one from it reflecting the photon and accelerating it away)."

http://www.discoversolarenergy.com/solar/sails.htm

The photon is reflected essentially unchanged, it still has the same energy.  So where has the energy imparted to the sail come from?

If we bounce light backwards and forwards between two parallel mirrors it does not loose energy by becoming red-shifted and yet the mirrors get a slight push on every reflection.

Here is another article on solar sails.
"Sailing Motion
Sailing motion is determined by theta, the angle between the solar radial and the ship's total force vector.  A positive theta generally adds energy and causes outward motion.  A negative theta generally reduces energy and causes inward motion.  The projection of the acceleration vector onto the velocity vector determines the actual change in energy in the local gravitational field.  A sail does not extract energy from or put energy into the reflected light to accomplish its sailing.

If the reflection does not extract energy from or inject energy into the photons, how does the sailing ship gain or lose energy? The reflected photons have the same energy flux they had prior to the interaction, but a different momentum vector. It is this altered momentum vector that gives the ship an accelerating force that allows it to work against the gravitational field to gain or lose energy within the field. The absorbed photons are the energy lost from the impinging flux. The absorbed energy is re-radiated from the sail, with some helping and some hindering the ships motion."

http://sail.quarkweb.com/light.htm

I believe this is saying that the altered momentum vector of the photons is where the force that causes the acceleration comes from.  In other words the force comes from the change in direction (through rebound) of the photons (Newtons third law...)  That still doesn't explain why the sail can gain energy if the reflected photons don't loose any.

Does anyone know of any experimental evidence confirming that reflected photons do exert a 'push'?
 

Offline simplified

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In what way is a reflected photon degraded?
« Reply #13 on: 06/11/2011 12:07:59 »
p=mc
2p=2mc
(2mc/mc)*100%≈0% of energy  :P
 

Offline JP

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In what way is a reflected photon degraded?
« Reply #14 on: 06/11/2011 14:20:54 »
Mike, if the photon gives up some of it's energy, it will rebound with a smaller frequency.  Generally they give so little energy to atoms or molecules that it's negligible.  With a solar sail, radiation pressure is also tiny, but there is a huge number of photons bouncing off it for a long period of time and no friction to slow it down, so these tiny pushes keep adding up.

The push of reflected photons can be measured, and I know of a bunch of experiments that measure it in one way or another, usually in a lab with laser light.  There is already one functioning solar sail, as well: http://en.wikipedia.org/wiki/IKAROS
 

Offline MikeS

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In what way is a reflected photon degraded?
« Reply #15 on: 07/11/2011 09:03:20 »
Mike, if the photon gives up some of it's energy, it will rebound with a smaller frequency.  Generally they give so little energy to atoms or molecules that it's negligible.  With a solar sail, radiation pressure is also tiny, but there is a huge number of photons bouncing off it for a long period of time and no friction to slow it down, so these tiny pushes keep adding up.

JP
I understand all of the above but I don't understand how a reflected photon can give up none of its energy (see my last post above) but still give a small push to the reflecting surface.  This would seem to imply an increase of energy which can't be the case as it would break the law on conservation of energy.

This makes me question whether a reflected photon can impart any momentum to the reflecting surface, or is it only absorbed photons that can do this?

Any experiment to measure the push of a reflected photon must involve a significant number of photons and can we be certain that all the photons have been reflected not absorbed?  As no mirror is perfect we can't be certain.  If an experiment were able to confirm that the push was twice as much for a reflected photon as an absorbed photon then it would be evidence confirming the hypothesis.  I have not been able to find any evidence of such an experiment.  If you know of any links I would appreciate you posting them.

It's been proven that solar sail technology is possible and we are told it is due to the push given by reflected photons bouncing off the mirror and the mirror recoiling but how can we be certain that it is not due to absorption? 

(Aside.  If it is from absorption then there could also be a small contribution from the forces that power a Crookes radiometer.  Presumably (?) if a solar sail works by reflection then it is not a heat engine but if it works by absorption then it is.)

Thanks for your time.
 

Online yor_on

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In what way is a reflected photon degraded?
« Reply #16 on: 07/11/2011 09:51:41 »
Well, yeah :)

I'm afraid I still think of it as a 'duality', although I can see why people jump from a photon to 'frequency', it's not the same to me. It's a weird subject though.

Both answers correct in century-old optics dilemma.
 

Offline simplified

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In what way is a reflected photon degraded?
« Reply #17 on: 07/11/2011 14:03:54 »
Mike, if the photon gives up some of it's energy, it will rebound with a smaller frequency.  Generally they give so little energy to atoms or molecules that it's negligible.  With a solar sail, radiation pressure is also tiny, but there is a huge number of photons bouncing off it for a long period of time and no friction to slow it down, so these tiny pushes keep adding up.

The push of reflected photons can be measured, and I know of a bunch of experiments that measure it in one way or another, usually in a lab with laser light.  There is already one functioning solar sail, as well: http://en.wikipedia.org/wiki/IKAROS
I know when the solar sail can be very effective.We should send a rocket to a comet.Then the rocket should travel on the comet till the maximal speed of the comet. Then the rocket should fly up from the comet and release a solar sail. A high-speed object receives more energy of photons, therefore such rocket can travel to the nearest stars. :)
Maybe the rocket can recieve the same speed without comet. :-\
« Last Edit: 09/11/2011 12:56:39 by simplified »
 

Offline JP

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In what way is a reflected photon degraded?
« Reply #18 on: 07/11/2011 14:13:57 »
Mike, if the photon gives up some of it's energy, it will rebound with a smaller frequency.  Generally they give so little energy to atoms or molecules that it's negligible.  With a solar sail, radiation pressure is also tiny, but there is a huge number of photons bouncing off it for a long period of time and no friction to slow it down, so these tiny pushes keep adding up.

JP
I understand all of the above but I don't understand how a reflected photon can give up none of its energy (see my last post above) but still give a small push to the reflecting surface.  This would seem to imply an increase of energy which can't be the case as it would break the law on conservation of energy.


It should give up some of its energy, and its frequency does change.
 

Offline MikeS

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In what way is a reflected photon degraded?
« Reply #19 on: 09/11/2011 09:23:29 »
If light bouncing back and forth between two mirrors looses its energy (as generally believed) by becoming red-shifted then this should be visible and measurable.  But in multiple reflections there does not appear to be any red-shift, instead the light gradually becomes dimmer.  This would seem to indicate that that the light is not loosing energy by being reflected but by absorption as the mirrors are not perfectly reflecting.

If anyone knows of any links to experimental evidence, one way or another, please post them.  Thanks

My own feelings are that reflected light does not impart momentum to the mirror and so is not red-shifted.
Light reflected by two perfectly reflecting mirrors would essentially last unchanged indefinitely, were it not for photons escaping through quantum tunnelling.  I appreciate this view may be wrong but would like to see the evidence.

More confusion here
http://www.physicsforums.com/archive/index.php/t-3647.html
 

Online yor_on

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In what way is a reflected photon degraded?
« Reply #20 on: 09/11/2011 10:41:40 »
I don't think there ever will be perfectly reflecting mirrors Mike. But assume there was, then they would have to 'reflect' perfectly too. Maybe that's the reason there can't be :)

Anything interacting with something else must impart a momentum, and lose some of its 'energy' in form of? Heat? and maybe 'something else' too? I don't know there. I'm still too hung up on the idea of usable 'energy' versus 'used unusable energy' to really make up my mind on that one. But if it 'interacts' it will lose 'energy'.
 

Offline simplified

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In what way is a reflected photon degraded?
« Reply #21 on: 09/11/2011 12:52:04 »
If photon gives more momentum to a mirror,then this photon gives less energy to the mirror. ;)
 

Offline JP

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In what way is a reflected photon degraded?
« Reply #22 on: 09/11/2011 13:25:16 »
Mike, I can't tell you about any experiments but I can tell you that energy and momentum are conserved.  That's a fundamental law, even in quantum mechanics.

Light does impart momentum to the mirror by conservation of momentum.  If the mirror moves as a result of that momentum, then the photon has to redshift.  I suspect this redshift can be thought of as the doppler shift, since the reflected photon is coming off a moving mirror.  Obviously the amount the mirror moves from one photon is infinitesimal, so the redshift is as well.
 

Offline simplified

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In what way is a reflected photon degraded?
« Reply #23 on: 09/11/2011 14:05:46 »
Mike, I can't tell you about any experiments but I can tell you that energy and momentum are conserved.  That's a fundamental law, even in quantum mechanics.

Light does impart momentum to the mirror by conservation of momentum.  If the mirror moves as a result of that momentum, then the photon has to redshift.  I suspect this redshift can be thought of as the doppler shift, since the reflected photon is coming off a moving mirror.  Obviously the amount the mirror moves from one photon is infinitesimal, so the redshift is as well.
Yes,then imparted momentum < 2p
 

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In what way is a reflected photon degraded?
« Reply #24 on: 09/11/2011 14:43:25 »
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.

"In empty space, the photon moves at c (the speed of light) and its energy and momentum are related by E = pc, where p is the magnitude of the momentum vector p. This derives from the following relativistic relation, with m = 0

    E2 = p2c2 + m2c4."

Also take a look at How Does the Total Energy of a Particle Depend on Momentum. 
=

Which then, if we apply it on the concept of a time less, mass less, photon implies that photons then can 'change energy' intrinsically :) (when 'reflected' from a mirror) I don't think so.

When we instead look at it as a wave?

"Minkowski's formulation, on the other hand, seems more natural from the point of view of quantum mechanics. As light slows down inside a medium its wavelength also decreases, but quantum mechanics tell us that shorter wavelengths are associated with higher energies, and therefore higher momenta. In fact, Minkowski's approach suggests that the momentum of a single photon of light increases by a factor n as it passes through a medium. This result can also be supported by strong theoretical arguments, among them one that considers what happens when an atom moving at some speed through a medium absorbs a photon and experiences an electronic transition."

And as a wave it should also interact, as it will pass glass in and back out, and so get a 'higher momentum' :) Eh, in a ordinary mirror that is.

But assume just a (perfectly) reflecting surface, no glass involved. Then, if it is true that wave can't lose 'energy' due to reflection we can let it reflect between two mirrors into infinity, so, will it lose momentum then? If we now assume that momentum is different from the concept of 'energy'?

Einstein used E=mc2, there mass is assumed to be 'E = energy'. Which in physics is the same 'm'  you multiply the velocity by, to find a momentum (p). Light use "m" because of its energy, not (invariant) rest mass.
« Last Edit: 09/11/2011 15:41:46 by yor_on »
 

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In what way is a reflected photon degraded?
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