Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: greeniemax on 27/02/2012 07:03:50
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I would really want to know how is light produced from an atom, for instance Hydrogen, if we heat hydrogen and it starts glowing it gives off light. Energy converted from heat to light.
I know that when energy is put in the system it excites electron and it jumps to higher orbit than it loses energy and moves back to lower orbit releasing a photon, I couldn't find information about how is that photon is produced. A mechanism, a process please. Not some magical reason for production of photon. There is no mechanism in electron jumping that could produce a photon, doesn't make sense to me.
Thanks
Mod edit - I've formatted the subject as a question, to help keep the forum tidy and easy to navigate.
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It's because an electron gives off an electric field. As the electron moves, it drags this field about with it, creating ripples that propagate away. These ripples are the light given off.
An analogy would be to think of a duck in a pond. A duck sitting still doesn't make waves. But if duck moves, it creates ripples in the water that travel away as waves.
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Great but that is explanation for how "light as wave" is created, I'm talking about single photon being created "photon as particle", the mechanism for it.
Wave explanation would be easy but single atom does not produce wave of light but a single photon of light.
Thanks
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The same idea holds for photons. The electron interacts with the EM field, so when it moves, it gives off photons. If it were to move continuously, it would generate smooth waves, but if it undergoes a quantum jump between energy levels, it gives off one quantum of EM energy: a photon.
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Sorry JP don't want to keep pushing on same topic over and over again but you see Photon is a particle and production of a particle isn't explained at all in what you are talking about.
Question is "mechanism of production of a particle (Photon) in an atom".
In my opinion, what I have learned from books and google search there is nothing on it, there is no mechanism defined for this process.
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Particle production is exactly what QED describes, and it does so in the way I pointed out above! If you don't think that's enough and want a theory that explains why QED exists, then you're out of luck. But physics never explains a root cause of everything and (probably) never will. So if you drill down deep enough, you'll always hit a point where we don't have a deeper theory.
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is that why pond water only ever comes halfway up a duck?
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I guess I'm out of luck than because I would like to know really how light is produced :(
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'Fields' are a very nice idea. And what's called 'excitations' relative 'relative motion' :)
Myself I find 'relative motion' creating a heck of a headache (enough for me at last) for the moment :)
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is that why pond water only ever comes halfway up a duck?
Bouyancy. (The duck is less dense than water). But that's got very little to do with light--I was just using it as an analogy about the creation of waves.
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Greenie,
I'll give it a shot to explain the "mechanism," though you may or may not like my answer. The basics deal with energy absorption into the atom and release back to a normal state. Any mechanism you like, heat, friction, collisions, impact by another photon, or any process can cause an electron to move from one state to another. The only requirement is that the energy impacted to the atom is the exact energy to move from one electron orbital to another. I think we're agreed on this from your first post. You are more concerned with the photon release from the atom as the electron moves back down from an excited state to a lower relaxed state.
The photon exhibits both wave and particle properties, but unlike the duck analogy, it is not a continuous wave. I have tried to draw a picture, but please forgive my mouse freehand drawing skills... If I stand in one place and watch the atom with the electron in an excited state, there is no wave produced. As long as the electron stays in the excited state, there is no wave eminating from the atom, it is just in a higher total energy state. When the electron decays to the relaxed state, a wave packet starts. When the electron hits the lower relaxed state, the wave packet ends. The energy in the photon is the exact same energy as the difference between the two energy levels, but the wave duration is only as long as it takes for the electron to move from the excited state to the relaxed state. If I watch the photon travel past me, there is nothing until the electron moves, then the amplitude of the wave packet increases, reaches a maximum, and goes back to zero as the electron hits the relaxed state again. Throughout the process, the wavelength or frequency of the photon remains the same. The amplitude does go from zero to a maximum, and back to zero.
That is where the particle duality side comes from. I hope this helps!
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I thought we couldn't see light till it hits something solid. If it excited electrons wouldn't the air around us glow?
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Yes and no, the air around us will not glow if the electrons are in an excited state. They will release a photon if the electron transitions between energy levels, but it will only release one photon per energy transition. Most of the energy transitions are frequencies outside the range humans eyes can see.
What you CAN see, and is evidence of the phenomena, is neon lights. In a neon tube, you put a gas in a sealed tube and then send electrons through the tube. As the electrons from the wall socket hit the gas molecules, electrons in orbit around the molecule are pushed into higher energy states. As the electrons fall back down into the relaxed state, they release the photon as stated before. With so many electrons flowing through the tube, the light appears constant, but it is really an enormous number of bound electrons transitioning from one energy level to another.
For very cheap you can find a diffraction grating on the internet. A diffraction grating will allow you to see the component wavelengths of light that make up whatever combined color emanating from the neon tube. Each line in your diffraction grating corresponds exactly to the energy difference between excited and relaxed states! It's worth the $3-4 on amazon to actual see it. But, here's a website that shows the effect.
http://diffractiongrating.net/
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Thanks Cheese for your great answer.
So basically what you are saying is that Photon is packet of waves? This would imply that light is a wave and not a particle at all as it doesn't exhibit any kind of mechanism to produce a new particle.
But if look at the QM description of electron as field of energy and that field changes its level from high to low it could give a bit of idea (like bubble on water creates sound waves) of production of wave. My problem is I can't see any mechanism that would explain light as pure particle, yes as you said packet of wave makes sense.
Thanks
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You have to be very careful when thinking about a photon as a packet of waves. It isn't a classical packet of waves, and doesn't have the properties of a classical packet of waves. For example, if I imagine moving an electron from one energy state to another classically, so that it crosses the space between the two energy states, it will release a short pulse of light. But this pulse can be divided in half with a beamsplitter and it can be broken into different color components with a prism. A single photon cannot be chopped in half and cannot be divided into colors (it consists of exactly one color).
To properly think about it, you can use the idea that an electron moving from one state to another will release EM energy, as Cheese2001 said, but you also need need to think of the jump between states as a quantum effect. A quantum jump between states can't be thought of as the electron moving through space between them. Rather, the electron can exist only in specific states and never in between. What happens in the transition from a higher to a lower state is that it becomes less and less probable to find the electron in the higher state and more and more to find it in the lower state. If you're watching the atom, at some point, the electron will disappear from the higher state and appear in the lower one. Because this is instantaneous, the wave released looks nothing like a short pulse you would expect if the electron moved continuously through the space between energy levels. This instantaneous jump, which is a purely quantum effect, gives the photon its odd particle-like properties, which are also a purely quantum effect.
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Sweet JP :)
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Hey! You're pretty good at this stuff JP. You might want to consider doing it for a living [:)]
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JP you may have to develop you mental model a bit because spectrum lines from atoms do have a "bandwidth" dependant on the lifetime of the state that is decaying to emit the photon. The process is not instantaneous. Very short lived states are more spread out in frequency than long lived metastable or "forbidden" states.
One of the questions that I have frequently asked experts and have never yet received a confident answer is whether bandwidth is a property exhibited by a single photon or is only observable by looking at a lot of photons from the same spectral source (because it is observable in those conditions). If it is, it means that a photon has more properties than just its frequency and this could have a fundamental effect on the way that we should develop theories.
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The term "photon" gets used loosely, but the precise term is that it's the smallest packet of energy of a monochromatic plane wave, so just like the plane wave that spawned it, it has zero bandwidth. Also, just like the plane wave that spawned it, physical fields generally get written as superpositions of photons. The photon's fundamental properties, by the way, are spin and wave vector, which are related to the polarization and wave vector of plane waves.
I gave a simple, hand-waving explanation above in order to give greeniemax some basics of why photon emission should be quantized, but it's not terribly rigorous, as you point out. I don't think the more rigorous treatment helps much with his question, so I didn't bother giving it. But here goes...
Consider a simple atomic model with two energy levels and one electron.
1) The electron starts off in the excited state and the surrounding field starts off with no photons. You have to consider the system of electron + EM field here.
2) The electron-field system evolves in time so that the electron can be in any superposition of its two states and the field can have any number of photons of any wave vector and polarization.
3) Since this is quantum mechanics, you need to dig into probabilities. You can compute the probability of each possible final state occurring at a given time from the initial state. This gives you the probabilities of photon emission.
4) You can consider the simple case of asking, "In a short time interval, what's the odds of a given photon being produced with a specified wave vector and polarization?" If you do this, you find out that a spectrum of photons may be produced, where the probabilities are governed by the starting atomic state. In particular, the time interval being considered is inversely proportional to the bandwidth of photons that can be produced.
(You can also be very general and consider photons in the EM field initially and an atom in any superposition of its two states initially and compute the probabilities of it being in different final states at different points in time.)
If you want the details (I'm not going to reproduce them), check out Optical Coherence and Quantum Optics, by L. Mandel and E. Wolf. It answers your questions in much greater detail. (Chapter 15 deals with photons interacting with atoms, and the equations I mentioned in the above description are all in sections 15.4 and 15.5).
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In light of all that, here's your confident answer:
A single photon has zero bandwidth. Bandwidth requires multiple photons. But physical processes that you would imagine produce only a single type of photon, such as a known atomic transition, actually produce a spectrum of photons due to finite time scales, so that as you measure many such transitions, you get a bandwidth. (Also, if you run the above arguments in reverse, it means that a photon doesn't have to be perfectly tuned to the atomic transition in order to be absorbed, so that if your detector clicks each time it absorbs a photon, that click has some finite bandwidth associated with it.)
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Hi JP great answer but there is a problem
Photo electric effect says that at time when Photon is if deep red color (almost infrared) it doesn't knock of any electron from metal. Problem is if light is photon and frequency of those photons gives it the frequency and if photon have 0 frequency, then it wouldn't matter they would still knock off electrons from material.
Secondly there is no mechanism described as to how this photon is produced, I could say that when I throw a stone in water it displaces some water depending on the size of the stone and this produces wave, there is no logical way it could be described that photon could be produced with the interaction of electron jumping from higher state to lower.
Should I consider that there is no mechanism at all that talks about this issue? Secondly please note that if photons are of 0 frequency than they wouldn't have color and would not be able to knock off or not knock off electrons based on color (as amplitude doesn't matter in this case).
Thanks man.
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JP that is by far the best answer that I have ever received and I accept it as full answer because it is totally logical and fully fits with classical and quantum views. The "zero" bandwidth definition also implies as with all other particles that all photons exist for all time throughout the whole universe it is only their "measurement" (by interacting with something else and creating a change) that defines their properties at that time.
To take things one step further. Our universe clearly appears to have a time limit at the beginning although it may not have one at its end. There is clearly at the moment a time bandwidth product for our universe (although because of inflation this may not be limited by the distance that light can travel since the big bang) This implies that an absolute zero bandwidth cannot exist. Now this is no problem for electromagnetic waves because they are on the whole quite high frequency but for gravitational waves this limit may prevent very slow things interactions happening in quite the same way because of the lower frequency limit. I have never seen any thoughts by cosmologists in this direction
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After reading what I have written I think I should re-explain.
If all photons are of 0 frequency and their frequency depends on distance between them as they are produced that means two photons are exactly same with no difference.
In which case it wouldn't make any difference what wavelength of light is used photoelectric effect will give same output at a particular amplitude of light. It wouldn't make any difference at different frequency as photoelectric effect was explained by Albert Einstein to give explanation of photon because it cannot be explained in classical way if we take light as wave.
So photons with 0 frequency isn't possible they have to have frequency to give it color otherwise its just like one crust of wave, there is no difference between a wave and particle theory of photon here.
Awaiting explanation of above.
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I'm not quite sure I follow, greeniemax. There's no such thing as a zero-frequency photon, since it would have no energy and not exist.
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So as Cheese said before "Photon is packet of Waves", now I can imagine how wave could be produced from an atom, but if photon is a separate entity than it seriously doesn't make any logical sense how a particle is produced from any defined process.
I think I'll go through Optical Coherence and Quantum Optics, by L. Mandel and E. Wolf.
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No. A photon isn't a packet of waves, but a packet of waves is made up of photons. It doesn't make logical sense if you don't understand the logic of quantum mechanics, unfortunately, since quantum objects don't follow the rules of logic that dictate how everyday objects/waves behave.
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I have mixed feelings about that last statement JP. I agree that the potential for a range of frequencies to be detected in a single given photon exists but to allow this a non linear detection process must be used. If a linear detection process is used the photon observed is always the same frequency as the source that generated it. If this was not the case spectroscopy would not be possible. This tends to imply that a photon is a single frequency wave packet.
The time duration of the detection process therefore defines the bandwidth of the observation
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You are right about that after studying QM for 5 years with different books (as I'm too old to go to college here), I find QM doesn't make logical sense from the day I started till today, read and heard lots of books, watched many lectures on iTuneU but seriously either my mind is not ready to accept it as logic or people just call it logic when its not at all.
Personally in my opinion logic should be something that make sense to you and has a well defined mechanics, I really couldn't find any answer for production of light on internet, it just simply isn't clear, its like coming half the way and it ends, its actually like watching half the movie so either you don't know what happened in start or how it ended.
Maybe we don't have complete information and just trying to come up with a picture that cannot be understood or we are on wrong path and trying to correct it with twitches from side.
Overall I'm totally confused, but thanks for your efforts to try to make me understand, specially JP and Cheese.