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Author Topic: What is "light" pressure?  (Read 11300 times)

Offline sorincosofret

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What is "light" pressure?
« on: 19/05/2008 00:28:06 »
Light and electromagnetic wave pressure

Background and actual explanation
Light pressure was accepted from long time in physics as an explanation for comet tails formation. Later Bartoli deduced the existence of light pressure from thermodynamic considerations and Maxwell predicted the value of light pressure from his theory of electromagnetic phenomena.
Scientists were making great efforts to measure the value of light pressure, at beginning without success. Besides its small value, there were other perturbing effects which have influence on the experiment. In fact the first explanation given by Crockes to its light mill invented device was wrong due to these errors.
Light mill consists of four vanes each of which is blackened on one side and silvered on the other.  These are attached to the arms of a rotor which is balanced on a vertical support in such a way that it can turn with very little friction.  The mechanism is encased inside a clear glass bulb which has been pumped out to a high, but not perfect, vacuum. When sunlight falls on the light-mill, the vanes turn with the black surfaces apparently being pushed away by the light.
Maxwell accepted the initial explanation of Crookes, that rotation is produced as result of radiation pressure as predicted by his theory of electromagnetism.
Soon was realised (Arthur Schuster) that mill is not rotating according to theory prediction, but in a contrary direction. Light falling on the black side should be absorbed, while light falling on the silver side of the vanes should be reflected.  The net result should be twice much radiation pressure on the metal side as on the black. 
Other explanations were proposed some of them explaining partially the overall effect. Working at atmospheric pressure, light used in experiment rise the temperature of material and surrounding air, too.  As result of air heating, there is an air flow and this acts on the sheets like a blowing wind.
The non-uniform heating of the sheets, since the light shine only one side of the sheets, leads to different amounts of energy transfer to and up molecules of surrounded air. 
Lebedev in 1901 managed to surpass these difficulties and to measure the pressure that light exerts on bodies.  The main component of Lebedev's apparatus was a set of flat metallic sheets, some with black colour (so they were fully absorbing) while some others were almost fully reflecting, fixed on a light rod like in fig. 1.



Figure 1. Schematic device used for measuring the light and microwave pressure

 The system was put in a vacuum chamber.
 As a result the pressure on the reflecting surfaces is twice that exerted on black surfaces, and this difference lead to a torque on the rod so that the hanging spring of the rod is twisted. Thus the twisting angle is directly related to light pressure.
The results obtained by Lebedev showed that the light pressure value agreed with Maxwell's theory.
Later, the pressure exerted by light was calculated using the relativity theory on basis of E=mc2, and the momentum is p=E/c.
   
New proposed experiment


   Original experiment was made using visible light. Whatís happened if instead of light, microwaves are used?
   On basis of special relativity, the energy of photons in visible and microwaves are:
    Ev=hνv
    Em=hνm

     With actual interpretation a visible photon has a frequency νv≈1015 and in microwave up to νm≈1013, consequently the source of radiation must be 100 times powerfully for microwave in order to get the same effect.
Up today the vacuum technique, microwave generation and the material science has made huge progress.
The experiment is designed to measure the pressure exerted by microwave on a similar device like the one used by Lebedev as in fig. 1.
 A sheet is build from material with high microwave reflectivity coefficient (heavily doped polypyrrole films available described at 
http://www3.interscience.wiley.com/journal/104033896/abstract?CRETRY=1&SRETRY=0), and the other sheet is from a material with a high absorbance coefficient (Microtubules of polyaniline as new microwave absorbent materials described at
http://www3.interscience.wiley.com/journal/88513004/abstract?CRETRY=1&SRETRY=0). 
With this improvement and using a microwave source, similarly the pressure on the reflecting surfaces is twice that exerted on absorbing surfaces, and this difference should create a torque on the rod so that the hanging spring of the rod is twisted.
In practice the rod is not twisted and the experiment leads to a negative results.
The negative results are due to a specific propagation of electromagnetic waves and due to difference between electromagnetic waves and light.
   Maxwell predicted that when electromagnetic waves hits an object and is reflected, the wave pushes on the electrons in the surface of the object, which in turn push on the rest of the object.
Is the explanation reliable?
   Letís consider that electromagnetic waves hit an electron, in the first semi period, when the electric field E increases, as in fig. 2. In this case the electron will be attracted toward the direction of increasing electric field.



After a semi period, the electric field changes its direction, and of course the electron changes its direction of motion, too, like in fig. 3.


        Because the change of electric field direction is made more then 108  times per second the electron due to its inertia does not move at all.   
   Consequently an electromagnetic wave canít produce a macroscopic pressure at absorption or reflection. The detailed interaction between electromagnetic waves and matter will be described in further study. For the proposed experiment, it is necessary to emphasize that in case of microwaves, the rod does not twist.
Another case is for light which has a corpuscular nature and photons leave their momentum at absorption or reflection.

« Last Edit: 09/06/2008 10:22:47 by chris »


 

lyner

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Re: What is "light" pressure?
« Reply #1 on: 19/05/2008 08:55:35 »
Have you tried reading a basic Degree - level textbook? It will explain the phenomenon very well for you.
 

Offline sorincosofret

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Re: What is "light" pressure?
« Reply #2 on: 19/05/2008 09:32:09 »
Dear Sophie,
Please indicate me such basic-degree level textbooks where the difference between momentum or pressure of electromagnetic wave and light is treated.
Up to date, I have read only in the last year about 200 high level books and there is no mention about such difference.
Of course... the experiment is quasi forgotten...
Regards,

 

lyner

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Re: What is "light" pressure?
« Reply #3 on: 19/05/2008 13:13:34 »
There is an ancient book, which I read only last year (not cover to cover) in which he discusses just that. The Author is Kraus, I seem to remember. He should give you a good run for your money.
Another book - I happen to have it on my lap at the moment, actually - is  by Panofsky and Phillips ;"Classical Electricity and Magnetism", Published by Addison Wesley in 1955.
In my edition it is dealt with on page 191.
No doubt you will be able to deal with the Maxwell Stress Tensor idea and point out where they have gone wrong?
I am sure that there would be no objection if I scanned in a couple of pages and posted them. Are you interested?
« Last Edit: 19/05/2008 13:15:27 by sophiecentaur »
 

Offline sorincosofret

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Re: What is "light" pressure?
« Reply #4 on: 19/05/2008 14:25:23 »
Please post the scanned pages here. In this way we see if we discuss about the same idea. I stress you that, to date, I haven't seen in any book the experiment made with electromagnetic wave (microwave) and the negative result obtained in this case. Of course the experiment with light is reminded in every medium to high level book so it's not worth to be discussed again.
As I remember I've read Panofsky and Phillips  and there is no indication about the extension of Lebedev experiment to microwave. It is supposed as default that all photons from radiowave to gamma wave must have the same comportment, more precisely to give a positive results when the pressure is measured.
So I wait with interest the scanned pages...
Regards,
 
 

lyner

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Re: What is "light" pressure?
« Reply #5 on: 19/05/2008 16:52:51 »
The scans are too big for these forum pages - I can email them if you let me have an addy.

btw, have you a relevant link for Lebedev? There are so many Google hits that I have no idea where to look. What is his contribution to the topic? He seems to have been a computer pioneer.
 

Offline Bored chemist

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Re: What is "light" pressure?
« Reply #6 on: 19/05/2008 19:12:22 »
Sorin, You say there's an experiment "In practice the rod is not twisted and the experiment leads to a negative results. ".
Can you provide details of who did this experiment and where the method and results are published please?

There's another problem with your idea. If th electrons can't keep up with microwaves at a gigahertz or so (and therefore give an average effect of zero), how do the keep up with light where the frequency is a lot higher?
 

lyner

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Re: What is "light" pressure?
« Reply #7 on: 19/05/2008 22:52:53 »
I think this guy is on another planet, actually.
 

Offline sorincosofret

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Re: What is "light" pressure?
« Reply #8 on: 20/05/2008 00:02:05 »
Dear Bored chemist,
The microwave has a no influence (at macroscopic level) on the electron motion.
The light photons which are not waves, but let's say small balls, leave their momentum when they are absorbed or reflected on a material.
Why is not these experiments in a scientific paper ?
Ask any scientific journal with a impact factor greater then 1,5 and you will find the answer.
On the site when I will have time, there will be a section with amazing answer why the submitted papers are refuted to appear in a scientific journal.
 

Offline sorincosofret

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Re: What is "light" pressure?
« Reply #9 on: 20/05/2008 00:05:51 »
For Sophie,
When I will have time I will post for you some simple 10 euro experiment, home made, for helping you to differentiate between physics and mathematics.
Please make comments on the topic and try to not divagate...

« Last Edit: 20/05/2008 00:07:49 by sorincosofret »
 

lyner

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Re: What is "light" pressure?
« Reply #10 on: 23/05/2008 21:36:18 »
As you are clearly short of time, I might recommend using what time you have in reading around the subjects and learning something substantial about one of your favourites instead of squandering it in many directions and making so many ill founded and baseless assertions. Do a simple Google search on some of these topics that you claim have no evidence and you will find a range of knowledge and evidence.

For people to be convinced they need more than just promises of future revelations.
« Last Edit: 23/05/2008 21:38:27 by sophiecentaur »
 

Offline Bored chemist

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Re: What is "light" pressure?
« Reply #11 on: 24/05/2008 00:40:35 »
"The microwave has a no influence (at macroscopic level) on the electron motion. " is a baseless asstertion- clearly false in practice because the source of microwaves that most people are aware of ie the microwave oven produces microwaves from the motion of electrons.
"The light photons which are not waves"
yet, they produce interference patterns etc.
Again a baseless assertion which is clearly demonstrated to be false by common experience- the colours on a CD or the oil film on puddles for example.
 

lyner

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Re: What is "light" pressure?
« Reply #12 on: 24/05/2008 15:47:20 »
B-C
I fear you are bashing your head against a brick wall here. I should retire gracefully, if I were you, whilst you still have your sanity.
 

Offline Bored chemist

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Re: What is "light" pressure?
« Reply #13 on: 24/05/2008 22:10:53 »
There's only slightly better evidence of my sanity than of these crackpot theories.
However I generally prefer that science, rather than what could be politely called "speculation", gets the last word.
 

lyner

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Re: What is "light" pressure?
« Reply #14 on: 25/05/2008 00:22:44 »
I think that you may as well go outside and shout the last word at the Moon. I don't think this 'speculator' will learn anything of what you are writing.
 

Offline sorincosofret

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Re: What is "light" pressure?
« Reply #15 on: 28/05/2008 01:28:04 »
Instead of long and without sense discussion, maybe someone is interested in making this experiments....
 

Offline Bored chemist

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Re: What is "light" pressure?
« Reply #16 on: 07/06/2008 18:22:40 »
Good idea.
I don't have a microwave oven (no real reason- I just don't think that the food tastes as good microwaved).
I'm sure that someone out there has and is prepared to undertake this experiment.
Please use the oven to cook some food.
That's it- not a very complicated experiment and one that will fit into many people's daily life without difficulty.


Now, since the microwaves are produced by resonance in a magnetron it proves that the electrons in that magnetron influence and are infuenced by the microwave radiation.
This clearly contradicts Sorin's theory that "The microwave has a no influence (at macroscopic level) on the electron motion."

Please post a reply if you are able to use a magnetron based microwave oven (I think they all are) to cook food.
 

Offline sorincosofret

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Re: What is "light" pressure?
« Reply #17 on: 07/06/2008 20:23:22 »
Bored chemist,
It's a dream the fact that in a common alternate electric current a electron is forced to oscillate at 50 HZ/second.
How can be true that a microwave field produce an oscillation of a electron.
And even in case of a electron oscillation, in a semi-periode electron is mowing away and in the second semi-periode the electron is moving contrary.
At macroscopic level should be no motion of electrons.
Another case is for a particle which hit the solid body only in a direction and the effect can be observed.

 

Offline lightarrow

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Re: What is "light" pressure?
« Reply #18 on: 07/06/2008 20:52:28 »
Original experiment was made using visible light. Whatís happened if instead of light, microwaves are used?
   On basis of special relativity, the energy of photons in visible and microwaves are:
    Ev=hνv
    Em=hνm

     With actual interpretation a visible photon has a frequency νv≈1015 and in microwave up to νm≈1013, consequently the source of radiation must be 100 times powerfully for microwave in order to get the same effect.
Why? Pressure is proportional to momentum p which, as even you have written is E/c. If the energy is the same, the pressure will be the same. What is different is just the number of photons hitting the target per unit time (100 times in the case of microwave).
« Last Edit: 07/06/2008 20:55:01 by lightarrow »
 

lyner

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Re: What is "light" pressure?
« Reply #19 on: 07/06/2008 22:06:28 »
sorin. You've done it again
Quote
At macroscopic level should be no motion of electrons.
Electrons are not on a macroscopic level. Did you know, they're really tiny!
 

Offline sorincosofret

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Re: What is "light" pressure?
« Reply #20 on: 07/06/2008 22:50:49 »
Ok Sophie,
I reformulate:
Even in case of electrons oscillations (why not nucleus oscillations too!?) the effect at macroscopic level it should be zero. I hope is a more clear English formulations. There are some differences between Latin and English topic of sentence ... so sorry for my English.
 

Offline Bored chemist

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Re: What is "light" pressure?
« Reply #21 on: 08/06/2008 13:35:19 »
Sorin.
you say that alternating electic fields cannot make electrons oscilate. "How can be true that a microwave field produce an oscillation of a electron."
You say it even after I have pointed out that the resonance in magnetrons relies on this.
OK how about particle accelerators like cyclotrons?
the people who use them say they work by accelerating electrons (sometimes other particles) back and to.

What unworldly explanation are you going to give for this?
Or are you simply going to ignore the fact that what you say is clearly wrong?
You were wrong about the magnetic field from a gas carrying a current.
You were wrong about the field from a semiconductor carrying a current.
You didn't understand how electric current flows through an electrolyte.

How many other things are you going to be wrong about befoer you accept that your "theory" is useless?
This isn't a language problem. The problem is your "theory".
 

Offline sorincosofret

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Re: What is "light" pressure?
« Reply #22 on: 08/06/2008 21:39:39 »
B.C,
I'm not interested to respond at your comments.
Please make a mental control learn to speak polite and after maybe I will answer to you.
 

lyner

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Re: What is "light" pressure?
« Reply #23 on: 08/06/2008 23:39:04 »
The force on an electron may be small due to a varying em wave. But, if you accept that a low frequency varying E field will make an electron oscillate, you cannot say that' at a certain frequency which is high enough' the movement will be zero. It will just be very small. Of course, the mean displacement will be zero, but that's not what counts for reflection to occur due to electron conduction - it's the vibration.
Using the accepted (by you,too, I hope?) value of  e/m = 1.75888e11 C/Kg, the force on an electron in a field of 1V/m  would produce an  acceleration of  about  1.7e11m/s/s.
Making a simple assumption that the em wave is a squarewave rather than a sinewave and using a typical frequency for light of 5e14Hz, the acceleration in each direction is acting for
0.5/5e14s = e-14s
By the end of each half cycle, the electron will have reached a velocity of
(Acceleration times time)
a = 1.7e11 X e-14
     = 1.7e-3m/s
This is roughly the speed of the electron over half a cycle (1.7mm/s)
Using an equation of motion (school stuff)
distance traveled, d, is 1/2 X acc X time (a half cycle, in this case)
d = 1.7e-3 X e-14
d = 8.5e-18m (during each half cycle)
Assuming that the effective 'field strength' of a photon of visible light is of the same order of magnitude as the electron volt energy  of the photons - say 2V
The 'wobble' produced by a 2V/m  would be about 1.7e-17m.  The classical diameter of the electron is about 6e-15 so the movement corresponds to about 0.3% of the diameter of an electron. That's not ZERO.
For a frequency of  10GHz, the time of a half cycle would be much greater and the distance moved would be greater - more like the diameter of an atom - easier to understand, perhaps, but still the same story basically.

Someone please check my arithmetic.
« Last Edit: 08/06/2008 23:43:48 by sophiecentaur »
 

Offline Madidus_Scientia

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Re: What is "light" pressure?
« Reply #24 on: 09/06/2008 06:24:56 »
B.C,
I'm not interested to respond at your comments.
Please make a mental control learn to speak polite and after maybe I will answer to you.

Seems like pretty good control under the circumstances. How conveinient though that you choose to ignore BC for poking holes in your logic.
« Last Edit: 09/06/2008 06:27:01 by Madidus_Scientia »
 

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Re: What is "light" pressure?
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