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Absorbtion of light- a dillema

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sorax123
#1
Nov9-11, 05:33 PM
P: 31
I am fascinated light and how its phenomena are possible, however one particular area in which there is slight doubt in my mind is in absorbtion, after reading both online and in books 2 conflicting theories on this.
The first idea is that a photon of light which has the correct amount of energy needed to make a molecule become excited into a further energy level, interacts with such a molecule and causes the molecule to become excited. After this point, if no further energy is given to the particle, it will lose energy and will no longer be excited, resulting in the electron dropping to a lower energy orbit and a photon of the same energy being reemited. Now this is all fine, but the flaw lies in the final part: if a photon of the same energy is reemitted, then surely there is no net energy gain, and therefore no absorbtion present?
The second theory relies solely on wave theory and says that a particle has a particular vibratioanl frequency at which it exists and if a light wave happens to be this wavelength or a discrete multiple of it (ie. 2x it or 3x or 4x etc), then it will be absorbed and a resonance effect will take place, resulting in more vibration in the particle and therefore thermal energy, explaining why a black object gets hot in the sun. But this does not seem right to me as it bases its argument on newtonian mechanical waves rather than electromagnetic fundamentals, surely the physical vibrational frenquency cannot intertwine with E.M fields? And if this theory were true then it must say that as a particle has more thermal energy (higher frequency vibrational frequency) then it changes colour?
Also, while I write this, I thought I'd pose the question, how is it possible for a substance to absorb both red and blue light, while not the colours inbetween, as blue light and red light are not linked by a discrete coefficient as suggested in theory 2?
Just looking for some clarifications here as I don't want to research further into these
subjects without full understanding of this seemingly illusively understood and debated principal.
Thanks in advance.
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fdangelis
#2
Nov9-11, 06:43 PM
P: 4
Quote Quote by sorax123 View Post
surely the physical vibrational frenquency cannot intertwine with E.M fields? And if this theory were true then it must say that as a particle has more thermal energy (higher frequency vibrational frequency) then it changes colour?
i think they can... otherwise microwave ovens wouldn't work.
and i don't know about changing colour... but they sure light up in high temperatures...

i have no idea about the answers to your questions, but wanted to leave some thoughts here... until someone comes with an answer.
sorax123
#3
Nov10-11, 10:56 AM
P: 31
Thanks for your reply. I'm pretty sure that micro waves heat food because water is a polar molecule and the oscillating magnetic and electric fields of a microwave cause the polar molecule to rotate and "bump" into other molecules, passing on heat energy. This only occurs for frequencies of around 2.4 GHz for water as this is the frequency at which it takes the correct time for the electromagnetic field to change from positive to negative and therefore rotate the molecule. This means the water molecule can achieve the fastest possible rate of rotation. Perhaps visible light behaves similarly, but then things would get exceptionally hot, so I'm not sure.
But for microwaves heating food, it's not the vibrations interacting it's the idea of polar molecules and the idea that one side is negative and another is positive causing repulsion and attraction and rotation.
Cheers.
D

Drakkith
#4
Nov10-11, 09:44 PM
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Absorbtion of light- a dillema

Quote Quote by sorax123 View Post
Now this is all fine, but the flaw lies in the final part: if a photon of the same energy is reemitted, then surely there is no net energy gain, and therefore no absorbtion present?
Why would there be no absorbtion? The atom or molecule can stay in an excited state for an extended amount of time. While it is excited it has the energy gained from the photon. Upon emission of the photon it loses the energy.
ZapperZ
#5
Nov10-11, 09:52 PM
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ZapperZ's Avatar
P: 29,238
Quote Quote by sorax123 View Post
I am fascinated light and how its phenomena are possible, however one particular area in which there is slight doubt in my mind is in absorbtion, after reading both online and in books 2 conflicting theories on this.
The first idea is that a photon of light which has the correct amount of energy needed to make a molecule become excited into a further energy level, interacts with such a molecule and causes the molecule to become excited. After this point, if no further energy is given to the particle, it will lose energy and will no longer be excited, resulting in the electron dropping to a lower energy orbit and a photon of the same energy being reemited. Now this is all fine, but the flaw lies in the final part: if a photon of the same energy is reemitted, then surely there is no net energy gain, and therefore no absorbtion present?
The second theory relies solely on wave theory and says that a particle has a particular vibratioanl frequency at which it exists and if a light wave happens to be this wavelength or a discrete multiple of it (ie. 2x it or 3x or 4x etc), then it will be absorbed and a resonance effect will take place, resulting in more vibration in the particle and therefore thermal energy, explaining why a black object gets hot in the sun. But this does not seem right to me as it bases its argument on newtonian mechanical waves rather than electromagnetic fundamentals, surely the physical vibrational frenquency cannot intertwine with E.M fields? And if this theory were true then it must say that as a particle has more thermal energy (higher frequency vibrational frequency) then it changes colour?
Also, while I write this, I thought I'd pose the question, how is it possible for a substance to absorb both red and blue light, while not the colours inbetween, as blue light and red light are not linked by a discrete coefficient as suggested in theory 2?
Just looking for some clarifications here as I don't want to research further into these
subjects without full understanding of this seemingly illusively understood and debated principal.
Thanks in advance.
You might want to start by reading the FAQ subforum in the General Physics forum, especially on the photon transport in solids.

Zz.
Claude Bile
#6
Nov11-11, 06:32 AM
Sci Advisor
P: 1,474
Quote Quote by sorax123 View Post
The first idea is that a photon of light which has the correct amount of energy needed to make a molecule become excited into a further energy level, interacts with such a molecule and causes the molecule to become excited. After this point, if no further energy is given to the particle, it will lose energy and will no longer be excited, resulting in the electron dropping to a lower energy orbit and a photon of the same energy being reemited.
This is not true in general. Molecules possess rotational and vibrational energy levels in addition to electronic energy levels and can exchange energy via non-radiative processes such as collisions. In a solid, the atoms cease to possess discrete energy levels and instead possess continuous energy bands. Again, energy can be exchanged via non-radiative processes; in solids this tends to be dominated by the exchange of vibrational energy (i.e. phonons).

Claude.


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