Where does light go?

[tuzzer]

Does light undestroyable? and it will just keep reflecting to different place?
Or will it be absorbed by something? Or will light just dim out on its own?

[silkworm]

If a photon disappears it's because it's absorbed by a material. A photon is a discrete packet of energy and makes whatever material it's being absorbed by vibrate, the magnitude of the vibration dependent on it's wavelength, aka color (in the visible spectrum). It's energy, and when it's spent, it's spent (meaning the energy is transfered to the material it's being absorbed by and no more photon). Now absorbing photons can make a material emit photons, but I think that may be outside of the realm of your question as it has nothing to do with the general "death" of a photon.

I hope that answers your question.

[5614]

Light does get absorbed as silkworm says.

Also light spreads out. For example in a light bulb all the light starts in the bulb but spreads out across the whole room. So if you put the light bulb at the end of your garden then the light would have to spread out all over your garden and your house and maybe a bit into your room (if your room was at the back of the house with a window), this will make it seem a lot dimmer. The bulb still produces the same amoutn of light, but it spreads out a lot more so seems dimmer.

Across massive distances light spreads out so much that it can become so dim that you cannot see it. In reality there is still a tiny bit of light coming from that bulb to you, just not enough for your eyes to see it. Theoretically even if the light bulb was an infinite distance from you then there would still be a bit of light going straight to you, but again it would be far too little to see with your eyes.

[tuzzer]

But where do those absorbed photon go? Would the Universe eventually be full of light?

And Why can't you fill up and pop a box with light?

[ecoli]

But where do those absorbed photon go? Would the Universe eventually be full of light?[quote]
It turns into energy, which is represented by the molecular vibrations

[quote]And Why can't you fill up and pop a box with light?

because photons are massless... they don't take up three demensional space.

[JustStuit]

In a similar related concept, when something emits light does the number of photons emitted depend on the energy used to emit them? If that makes sense.

[RyanJ]

In a similar related concept, when something emits light does the number of photons emitted depend on the energy used to emit them? If that makes sense.

Yes an no, photons come in distinct energy groups, you can't just have any energy value for a photon, the groups are the only valid energy levels.

Thats the way I understand it, not shure how accurate that statement is :-)

As for light being absorbed, I think of it this way:

A photon is a concept to represent light as energy in a particle form, when its absorbed the energy of it and the thing it joins with unify becomming one. There never was a problem because the photon is simply a concept in this case to represent the energy trasfered, make sence to anyone?

Cheers,

Ryan Jones

[swansont]

In a similar related concept, when something emits light does the number of photons emitted depend on the energy used to emit them? If that makes sense.

As RyanJ said, yes and no: it depends on the circumstances. In a single atomic transition, the energy is related to the frequency, but you will get only a single photon. More energy in the transition will give you a higher-energy photon, but not more photons, for a single interaction. But having more energy by having more atoms in an excited state will give you more photons.

[5614]

because photons are massless... they don't take up three demensional space.Although photons do have a momentum so when they hit a wall they will exert a force on it.

[swansont]

because photons are massless... they don't take up three demensional space.

However, you can be in a situation where you can't have a photon (of a particular wavelength) because the volume is too small; the cavity won't support the standing-wave mode. But once you satisfy the boundary conditions you can put as many photons into it as you want, within engineering limitations.

[silkworm]

But where do those absorbed photon go? Would the Universe eventually be full of light?

When the photon is absorbed the vibration it causes either causes friction and is lost as heat, or it can do useful work overcomming an activation barrier or decomposing a molecule. It sort of depends on the situation.

And no, that says nothing about the universe filling with light.

And Why can't you fill up and pop a box with light?

I don't know what you're saying here. If you're saying why can't you hold a photon in a box? Well, you can. Just not for very long. And it's more of a mirrored pocket than a box. If you're asking why can't you stuff photons in a box, and fill it up until it bursts, well, you can do that too, if you have material inside that decomposes to a gas when exposed to a particular area of the electromagentic spectrum.

In a similar related concept, when something emits light does the number of photons emitted depend on the energy used to emit them?

You can take Planck's equation E=hv and say that E/h = v, which says verbally Energy over Planck's constant = frequency. Wavelength, frequency, and energy are all intertwined.

[s pepperchin]

Does light undestroyable? and it will just keep reflecting to different place?
Or will it be absorbed by something? Or will light just dim out on its own?

All you need to know is that energy is conserved but it can be changed from one form to another. A photon is just one form that that energy can take. When light from the sun strikes something the light of the wavelength which matches the color of the object is reflected and the rest is absorbed where it is changed to other forms of energy. It could be heat, other EM waves, or electricity, etc. This is a simple conservation of energy question. As far as the box of light goes it sounds to me that you want a blackbody.

[Sashatheman]

i have a question.

If light can bounce of objects, doesnt it mean it has to reduce speed to zero and go in a negative direction? Which would mean light doesnt have a constant speed

[5614]

It is all an instant thing.

A photon travels at c until it is absorbed at which point it ceases to exist... when an electron jumps to a lower energy level it releases a photon, the photon has speed c as soon as it comes into existance, it keeps this speed until it ceases to exist (upon absorption).

[swansont]

When the photon is absorbed the vibration it causes either causes friction and is lost as heat

Friction is really a macroscopic property. The vibration is the manifestation of thermal energy.

[silkworm]

Friction is really a macroscopic property. The vibration is the manifestation of thermal energy.

I do a pretty decent amount of analytical chemistry. We use spectroscopy to stretch, bend, and make molecules and atoms vibrate by exposing them to sets of wavelengths in the electromagnetic spectrum. You can heat up molecules and make them vibrate, and you can also make the molecules vibrate, stretch, bend by exposing them to certain wavelengths and that energy eventually leaves.

[swansont]

I do a pretty decent amount of analytical chemistry. We use spectroscopy to stretch, bend, and make molecules and atoms vibrate by exposing them to sets of wavelengths in the electromagnetic spectrum. You can heat up molecules and make them vibrate, and you can also make the molecules vibrate, stretch, bend by exposing them to certain wavelengths and that energy eventually leaves.

And I use lasers to cool atoms down. Temperature is proportional to the kinetic or vibrational energy. But heat, or thermal energy, is not some separate phenomenon from this. Saying that a photon is absorbed and the energy is lost as heat implies (to me, anyway) that the photon absorption wasn't heat transfer in the first place, and that's wrong. It's all heat transfer, so that's too general a term to apply to the individual processes.

[Snail]

And I use lasers to cool atoms down. Temperature is proportional to the kinetic or vibrational energy. But heat, or thermal energy, is not some separate phenomenon from this.

What happens to photons when producing latent heat...they get absorbed, the kinetic energy stops rising, the bonds break to change state...then what. What happens to the photons after this point, or do they literally get fizzled out when breaking the bonds, or do they eventually get released through some other means ??

[swansont]

What happens to photons when producing latent heat...they get absorbed, the kinetic energy stops rising, the bonds break to change state...then what. What happens to the photons after this point, or do they literally get fizzled out when breaking the bonds, or do they eventually get released through some other means ??

Once the photon is absorbed there is no more photon. The energy is in some other form. Other processes can make new photons.

[Snail]

Once the photon is absorbed there is no more photon. The energy is in some other form. Other processes can make new photons.

Thanks Swansont. Although 5614 kinda already answered the question, I just wanted it clarified with regards to latent heat.

[VendingMenace]

When the photon is absorbed the vibration it causes either causes friction and is lost as heat, or it can do useful work overcomming an activation barrier or decomposing a molecule. It sort of depends on the situation.


Ignoring swansont's comments concerning heat (in which his is correct -- if you consider temp to be average kenetic energy). There are very few processes for which an increase in vibrational enegy level of a molecule will provide the nessesary activation energy. Most chemical processes (processes in which electrons move about) require much larger activation energies than several thousand wavenumbers. I am not saying that no such processes occur, just that most light that is only able to effect a vibrational transition is not energetic enough to effect a chemical transition. Most light that can significantly effect the rate of a reation is found in the visable range or higher (in energy).

[silkworm]

And I use lasers to cool atoms down. Temperature is proportional to the kinetic or vibrational energy. But heat, or thermal energy, is not some separate phenomenon from this. Saying that a photon is absorbed and the energy is lost as heat implies (to me, anyway) that the photon absorption wasn't heat transfer in the first place, and that's wrong. It's all heat transfer, so that's too general a term to apply to the individual processes.

E=hv. I can't really say anything more than that. I was trying to illustrate to the author of this thread that once a photon is absorbed that's not necessarily the end of it, but if that energy isn't doing anything it is lost as heat. I was trying to show that while it does not have mass it is still energy. I don't understand the issue here.

There are very few processes for which an increase in vibrational enegy level of a molecule will provide the nessesary activation energy. Most chemical processes (processes in which electrons move about) require much larger activation energies than several thousand wavenumbers. I am not saying that no such processes occur, just that most light that is only able to effect a vibrational transition is not energetic enough to effect a chemical transition. Most light that can significantly effect the rate of a reation is found in the visable range or higher (in energy).

Light has a very real effect on many chemical processes. Need a list?

[swansont]

E=hv. I can't really say anything more than that. I was trying to illustrate to the author of this thread that once a photon is absorbed that's not necessarily the end of it, but if that energy isn't doing anything it is lost as heat. I was trying to show that while it does not have mass it is still energy. I don't understand the issue here.


You were referring to vibrational energy and thermal energy as if they were two different things

[silkworm]

You were referring to vibrational energy and thermal energy as if they were two different things

You can heat up molecules and make them vibrate, and you can also make the molecules vibrate, stretch, bend by exposing them to certain wavelengths and that energy eventually leaves.

No, I didn't. Thank God that's all that was, I was getting worried because I generally have faith in you knowing what you're talking about. Just a miscommunication.

[VendingMenace]

Light has a very real effect on many chemical processes. Need a list?

If you can provide me with a list of chemical processes that are affected by light that excites purely vibrational/rotational transitions, then sure.

I will accept that given enough infrared light, one can heat up a reaction to the point that you have supplied the requisite activation energy, however,....



When the photon is absorbed the vibration it causes either causes friction and is lost as heat, or it can do useful work overcomming an activation barrier or decomposing a molecule. It sort of depends on the situation.


emphasis mine -- reading "the vibrations it causes can do useful work overcomming an activation barrier"

So, i would love to hear some examples where purely vibrational events provide the activation energy or lead to decomposition.

I am not saying that it doesn't happen, just that I know of no examples were it does...

[5614]

The reaction:
CH_4 + Cl_2 \to CH_3 Cl + HCl
requires UV light to overcome the activation energy barrier (ie. take place).

[VendingMenace]

The reaction:
CH_4 + Cl_2 \to CH_3 Cl + HCl
requires UV light to overcome the activation energy barrier (ie. take place).

You are correct.

However, UV light excites an electronic transition which is accompanied by a vibronic and rotational transition. I had ask for an example of a purely vibrational or rotational transition providing the requisite activation energy.

The only example that I can think of that even comes close might be in some isomerizations (ie. switching between chair conformations in a hexane molecule) however, this is much more a physical process than a chemical process, which is what I think we were discussing.

[5614]

However, UV light excites an electronic transition which is accompanied by a vibronic and rotational transition. I had ask for an example of a purely vibrational or rotational transition providing the requisite activation energy.
I was referring to where silkworm said "it can do useful work overcomming an activation barrier or decomposing a molecule".

reading "the vibrations it causes can do useful work overcomming an activation barrier"

So, i would love to hear some examples where purely vibrational events provide the activation energy or lead to decomposition.

Silkworm said "vibrations it causes", whereas you then added in the "purely" which changed the meaning. Silkworm failed to mention about rotation which could be misleading, but he didn't specifically state there was no rotational movement.

I don't know of an example with solely a vibrational or rotational transition, but I would certainly not make a conclusion about whether or not it is possible based on what I currently know.

[VendingMenace]

Silkworm said "vibrations it causes", whereas you then added in the "purely" which changed the meaning. Silkworm failed to mention about rotation which could be misleading, but he didn't specifically state there was no rotational movement.


At the risk of getting involved in an argument online, i will add this...

There are three basic types of transitions that light can excite in a molecule (at least as far as this thread is concnered).

1) Electronic
2) Vibrational
3) Rotational

These have been listed in order of descending energy. This means that the energy required to effect an electronic transition is less than that for a vibrational one, which is less than that for a rotational one. It also happens that electronic energy levels (the stationary states) have less energy than vibrations which have less energy than rotations.

The point being this; if vibrations do not have enough energy to overcome an activation energy barrier, then rotations certainly do not. Thus, my consideration of a purely vibrational transition is a simplification of a vibrational/rotational transition.

So silkworm's assertion that, "the vibrations it causes can do useful work overcomming an activation barrier" can be adressed by considering purely vibrational transitions. Since he is asserting that it is the vibrations that provide the activation energy, it is acceptable to isolate the vibrational motions and transitions and ask if they are able to provide the requisite energy for a chemical process. WHich is what I did.

I hope that makes some sense?


I don't know of an example with solely a vibrational or rotational transition, but I would certainly not make a conclusion about whether or not it is possible based on what I currently know.

Fair enough. I am not sure myself. I think it is highly unlikely, but I do not really know, which is why i ask silkworm what he was thinking of, since he made the above claim. Just curious is all, really.

[Norman Albers]

Yo, did you make a mistake? You say descending order but then say 'less'...

[5614]

I think you meant ascending order of energy (ie. energy getting bigger as you do down the list). So for energy: 1<2<3

And you've contradicted yourself, I think. You said that vibrational energy is less than rotational, but that "if vibrations do not have enough energy to overcome an activation energy barrier, then rotations certainly do not" which is illogical because rotational contains more energy, so could over the activation energy barrier.

So what are you trying to argue (without having an argument!) here? If for energy: electronic < vibrational < rotational then you could vibrational without roational (if there wasn't sufficient energy), but if you had vibrational then would you have to still have electronic?

[swansont]

I think you meant ascending order of energy (ie. energy getting bigger as you do down the list). So for energy: 1<2<3

And you've contradicted yourself, I think. You said that vibrational energy is less than rotational, but that "if vibrations do not have enough energy to overcome an activation energy barrier, then rotations certainly do not" which is illogical because rotational contains more energy, so could over the activation energy barrier.

So what are you trying to argue (without having an argument!) here? If for energy: electronic < vibrational < rotational then you could vibrational without roational (if there wasn't sufficient energy), but if you had vibrational then would you have to still have electronic?

Electronic transitions are highest energy of the three.

[swansont]

So silkworm's assertion that, "the vibrations it causes can do useful work overcomming an activation barrier" can be adressed by considering purely vibrational transitions.


It depends on what "it" is referring to in the claim. I took it to mean the photon, not the vibrational energy.

[5614]

Electronic transitions are highest energy of the three.Ah, ok, thanks.

So is there still a dependancy of one on the other? Like can electronic happen without vibrational and rotational?

Also as electronic is the highest then surely rotational could happen without vibrational if there is limited energy?

But could vibrational occur without the lower energy rotational?

[Norman Albers]

I am enjoying the same confusions over in: http://www.scienceforums.net/forums/showthread.php?t=13250

[VendingMenace]

lol...stupid me. I did mean to say "more" when i wrote "less" instead. I think swansont has cleared up this confusion. Wow, that was dumb (on my part).


It depends on what "it" is referring to in the claim. I took it to mean the photon, not the vibrational energy.


I guess...though post#11 makes it pretty clear that he was talking about the vibrations of the molecule.

Whatever. This discussion has way outlived its usefullness, i think it is clear that vibrations and rotations are usually not of the energy required for the breaking of a chemical bond or the overcomming of a activation energy...at least not anything that anyone has thought of yet.

THough, i have been thinking about this and there seems to be a few cases where this might not be the case...
1) If you count the temperature being raised in solution by vibrational and rotational transitions (ie. in a microwave).
2) Perhaps in a very unstable molecule like tetrazine vibrational excitment *might* lead to decomposition. But i really don't know.
3) In redox systems that are undergoing self-exchange of electrons, if the system is strongly coupled enough, then there can be a phenomenon called vibrational coupling, where the electron transfer event can depend strongly on the vibrational modes of the molecule.

I think that is it for now. At least that is what i have come up with so far...


To answer 5614;

Vibrational transitions can be accompanied by rotational transitions.

and

Electronic transitions can be accompanied by vibronic transitions.

[5614]

Vibrational transitions can be accompanied by rotational transitions.Yes I know they can... but do they *have* to? ie. could a vibrational transitions occur without being accompanied by rotational transitions?

Going back to the original question about solely vibrational or rotational transitions... surely it would be theoretically possible to have rotational transition only if there was not enough energy to induce a vibrational transition as well.

[Norman Albers]

Vending Menace, your last statement may be what I have been trying to get at.. My question was, how does visible light (alone, say) leave us warm???

[swansont]

Vending Menace, your last statement may be what I have been trying to get at.. My question was, how does visible light (alone, say) leave us warm???

Electronic excitation, followed by (some) nonradiative relaxation. Here is more (http://physics.memphis.edu/Research/BiomatlabLum.htm). And here is lots more (http://www.olympusfluoview.com/theory/fluoroexciteemit.html).

[Norman Albers]

Very nice, thank you!