# Lights interaction with matter

Farn

## Main Question or Discussion Point

[SOLVED] Lights interaction with matter

When light (photons/waves/whatever) comes into contact with matter, what are all the possible outcomes? Anyone mind listing them out with an explanation for each?

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Chi Meson
Homework Helper
This is a partial list with much detail (deliberately) left out.

1. The photon could be absorbed. The enrgy of the photon ends up as molecular motional energy (thermal) and temperature goes up.

2. The photon could be re-emmitted. If the energy of the photon does not match an electron energy-level jump, the photon will "come back out." THis is what happens when light goes through transparent material. THe absorption/re-emmission is what "slows" the progress o fthe light through the material.

3. The photon could be scattered. Sortof like the last one, but the re-emission is in a random direction, rather than the same direction.

4. Stimulated emission: if a photon hits an energized atom, where an electron is at a higher energy state, and if the energy of the photon is the same as the energy-level "jump" of the electron,then two idential photons will be emitted.

5. Flouresence: an atom absorbs a high-energy photon, but re-emmits several lower energy photons, where the total energy of the emmitted photons equals the energy of the absorbed photon.

um...there's more

Farn
Thanks for the response!

Few questions...
What's required for the photon to be absorbed w/o re-emission and go twords raising temp?
The last 4 basically all involve absorption and then some kind of re-emission. What controls the type of emission that takes place?

Chi Meson
Homework Helper
Two Words:

quantum probabilities.

A lot has to do with the electron energy-levels. If an atom or molecule has an allowed energy transition (means: "size of jump for electron")that is equal to the energy of the photon, then the electron can stay at that level for a long time(minutes) or a little time (nanoseconds) or something in between.

THe atom/molecule can be set into vibration, rotation, or linear motion which is manifested as "molecular motion" which is what thermal energy is. There are quantum rules that I have forgotten as well as those I never knew that allow us to determine the probability of each the various outcomes.

Infra-red light just happens to have the right energy level to cause lots of molecules to "resonate" because they have electron energy transitions that match. Therfore, IR radiation is felt as "heat."

Ultraviolet light is higher energy and there are lots of compounds that will absorb UV because the photon energy matches a large "multiple jump" for an electron, but when it descends, it takes several lower energy jumps, each emmitting lower energy light.

I still can't expand that list, but I know I'm leaving something out.

Chi Meson
Homework Helper
Oh yeah. Duh. reflection. When a photon hits a metal surface it encounters "free-electrons" which are not behaving the same as electrons that are stuck in energy-levels. THe difference between reflection and absorbtion/re-emmission or scattering, is that there is no polarization caused by metallic reflection.

And as I recall, those quantum rules are a bunch of mathematical wave formulas for which there are not many decent analogies that allow a mental picture of "why" these things happen.

Originally posted by Chi Meson
Infra-red light just happens to have the right energy level to cause lots of molecules to "resonate" because they have electron energy transitions that match. Therfore, IR radiation is felt as "heat."
Chi Meson,

I got chewed out a few weeks ago
for saying that electrons changing
orbit could radiate in the infra-
red range.Do you have any links
to a site that would constitute
this? The person here said the
infrared could only arise from
the bonds vibrating like springs,
and that only the larger atoms
the infrared.

-zoob

Farn
Chi,
from your list I assume what gives an object (lets say green paper)its color is the paper absorbing all the light, but then having to emit photons with energies in the green spectrum. Now it seems right (from everyday experience) that the paper will do a #3 and scatter the photons all about, but can it also do a #4 or 5?

Chi Meson
Homework Helper
Originally posted by Farn
Chi,
from your list I assume what gives an object (lets say green paper)its color is the paper absorbing all the light, but then having to emit photons with energies in the green spectrum. Now it seems right (from everyday experience) that the paper will do a #3 and scatter the photons all about, but can it also do a #4 or 5?
#4 is stimulated emission, which must happen sometimes in nature, but only by pure accident. But this is what happens in lasers, and in order for this to occur you have to get a bunch of atoms correctly "set up" to release a particular photon if they make a certain jump (a condition known as a "population inversion"). Then one of the atoms spontaneously makes the "jump", and if the photon hits another atom, then two identical photons are created, then they get two freinds, and so on, and so on...
Getting this population inversion set up is tricky, which is why you do not see lasers occuring in nature. SO will it happen in paper? probably a photon or two every now and then, but nothing significant, and again, purely accidentally.

#5 (flouresence) you can see by shining "black light" on white paper, or even better "flourescent" paper. An invisible UV photon is absorbed, but other visible photons are emmitted.

Chi Meson
Homework Helper
Originally posted by zoobyshoe
Chi Meson,

I got chewed out a few weeks ago
for saying that electrons changing
orbit could radiate in the infra-
red range.Do you have any links
to a site that would constitute
this? The person here said the
infrared could only arise from
the bonds vibrating like springs,
and that only the larger atoms
the infrared.

-zoob
Unbelievable.
I wonder where he thinks how IR comes out of the sun? THere are NO compunds on the sun, yet most of what it puts out is IR. (most photons, not most energy)

In the hydrogen atom (which is the farthest you can get from larger atoms) it's called the Paschen series. As the electron de-exites from higher energylevels down to the third level, all emitted photns are infra red. WHenever the difference in energy levels is less than approximately 1.7 eV, the emmitted photon will be infra red.

As for bonds vibrating like springs, this is a description of molecular energy (internal, or thermal energy). Sounds like someone has the two very different realms of "heat" mixed up.

I tried a google on Paschen series and got a few sites, but do a search for "Brackett Series" and you can find Brackett's own paper from 1922. (The Brackett series is for de-exitation down to the 4th energy level in the hydrogen atom, even deeper IR than Paschen)
http://dbhs.wvusd.k12.ca.us/Chem-History/Brackett-1922/Brackett-1922.html [Broken]

He doesn't know what he's talking about.

Last edited by a moderator:
Thanks, Chi Meson.

Before you take my side I should
clarify that this was in reference
to the heat given off by chemical
reactions.I explained this heat
by the mechanism of electrons
changing orbit as a result of the
general disturbances going on, a conclusion to which I had jumped because I was not aware at the
time that chemical bonds are sus-
eptable to vibration at infrared
wavelengths.

I do not know if I jumped to the
right or wrong conclusion, but
the reason he claimed it was
wrong was by saying the major-
ity of atoms simply do not have
orbitals that can ever correspond
to infrared wavelengths.

Thanks for finding the info for
series and brackets.
Much appreciated.

-Zooby

Last edited:
drag
Greetings !
Originally posted by Farn
When light (photons/waves/whatever) comes into contact with matter, what are all the possible outcomes? Anyone mind listing them out with an explanation for each?
Well, Chi Meson basicly mentioned all the main possibilities
and explained them. I just wanted to mention one I find
very cool - when a very high energy photon like X-ray
and higher up the spectrum passes near an atomic nuclei
it could become (I'm not sure of the right word to use here)
an electron and a positron - its anti-matter equivalent with
a positive electrical charge.

Live long and prosper.

Oh, you mean pair-production.

Chi Meson
Homework Helper
Originally posted by zoobyshoe
Thanks, Chi Meson.

Before you take my side I should
clarify that this was in reference
to the heat given off by chemical
reactions.I explained this heat
by the mechanism of electrons
changing orbit as a result of the
general disturbances going on, a conclusion to which I had jumped because I was not aware at the
time that chemical bonds are sus-
eptable to vibration at infrared
wavelengths.

I do not know if I jumped to the
right or wrong conclusion, but
the reason he claimed it was
wrong was by saying the major-
ity of atoms simply do not have
orbitals that can ever correspond
to infrared wavelengths.

Thanks for finding the info for
series and brackets.
Much appreciated.

-Zooby
OK. I retract saying that "he doesn't know what he's talking about." We are obviously in chemistry here, and I have no authority whatsoever. So considering what I do know: atoms, from hydrogen to uranium, can and do emmit IR photons. They must be "pumped" first though, to push an electron into a higher orbital from which it can jump down, so in chemical reactions I suppose it's unlikely that Hydrogen's electron would find itself in the 5th energy level and then decend only to the 4th or 3rd.

So...I'm gonna step out of this one

Originally posted by Chi Meson
So...I'm gonna step out of this one
Not a problem. I didn't really
mean to invite support for any
side so much as to clarify the
confusion he left me with in

So let me be more specific. Can
a laser emit infrared? If so, can
you get it to do this by pumping
it with infrared or do you need
to pump it with higher frequency
light to get infrared out? If a
laser can emit infrared is it
being emited by electrons falling
to lower orbits, or is the pumping
somehow stimulating emission by
causing the molecular bonds to
oscillate(eg:CO2laser)?

Thanks,

-zooby

Chi Meson