Why do certain frequencies of radiation react with some things an not others,

  • Level: Undergrad 
  • Thread starter Thread starter LogicalAcid
  • Start date Start date
  • Tags Tags
    Frequencies Radiation
Join the discussion
Registration is free. Start your own thread to ask a follow-up.
Sign up Log in
10 replies · 4K views
LogicalAcid
Messages
137
Reaction score
0
For example, why is it that sand stays hot even after the sun goes out, stopping the source that excites its electrons, shouldn't that make it immediately cold, or is the radiation released in a shorter amount of time, and if so how?
 
Physics news on Phys.org

Discussion

Have you noticed that when you cook food, it doesn't immediately get cold when you take it out of the oven? Or that when it gets warm out, snow does not immediately melt? Heat transfer takes time.

Also I don't really see how your question and the thread title are related.
 
It takes time for the energy of a system to change. It might help to get an intuitive feeling for what that energy really is; it's the kinetic energy of the molecules contained in sand. When rays of sun light hit the sand, some of the energy contained in the light is transferred into the sand, causing these sand molecules to "jiggle" and to feel hot to the touch. When the sun sets and no more rays of sunlight are available to keep the sand molecules "jiggling", the jiggling slows down over time, for a variety of reasons; some of that energy in the sand is transferred into the ground, into the air, etc...
 
mathman44 said:
It takes time for the energy of a system to change. It might help to get an intuitive feeling for what that energy really is; it's the kinetic energy of the molecules contained in sand. When rays of sun light hit the sand, some of the energy contained in the light is transferred into the sand, causing these sand molecules to "jiggle" and to feel hot to the touch. When the sun sets and no more rays of sunlight are available to keep the sand molecules "jiggling", the jiggling slows down over time, for a variety of reasons; some of that energy in the sand is transferred into the ground, into the air, etc...

So some things electrons remain in exited states for longer amount of times? Explain how this happens please.
 
Why does a spring require time to stop bouncing? Similar.
I don't remember ever studying this, but my best guess is: The heat energy is being transferred back and forth internal to the warm surface but the oscillations also transfer a certain amount to whatever is external to the warm material. Boing, Boing, Oops I just vibrated one of those external guys. Chuckle.
DC
Oh darn, I'm talking about conduction and you are asking about radiation. Only thing that occurs to me is that if all the electrons did drop "orbits" at one time you would have one hell of a IR Laser.
Somebody take this one please, I got to get some sleep.
 
Last edited:
LogicalAcid said:
So some things electrons remain in exited states for longer amount of times? Explain how this happens please.

It seems like you're equating the thermal radiation with a spontaneous emission process. It's not like the sand is a semiconductor with a band gap in the IR. The atoms in the sand have some kinetic energy, and over time they lose this by colliding with other atoms in the sand. Some of these collisions will result in the energy being released as radiation. You have charges accelerating, and this requires radiation.

Maybe there is a way to think of this process as a particle relaxing from an excited state, but that's not the typical way to think about it and I feel like it might be providing some misconceptions.

It's true that there is some concentration of thermally excited carriers in the conduction band, and this concentration will decrease as the temperature falls, but it's not a one way process - these carriers are constantly being excited and relaxing. This isn't responsible for the general radiation from an object, because if it were we would only have radiation in a small bandwidth around the energy of the band gap.
 
Black body Radiation
In essence the light produced from objects that make black body radiation comes from the electrons in atoms. As the electrons move from one orbit to another in an atom, they produce photons. This process is very well understood by scientists. The development of Quantum Physics started as a result of scientists efforts to explain Black body Radiation.

The above is a quote from a Unified Field Theory article online. Think photons.

Here is an interesting phenomena involving weather that I believe relates to the question of cooling. If you have a cloudy night, particularly a quiet layer, surface temperatures (of the sand) will not drop as much as on a clear night. The weather guys say the heat is trapped. What really is happening (mind blowing to me) is the photons from the sand are radiating to the base of the clouds, being reflected back and "reheating" the sand at the rate of a gazillion times a second. Back and forth all night. The absorption and radiation on both ends is a quantum process due to photon-electron interaction.

Seems to me that the atoms in the sand would be interchanging photons and shifting electron orbits throughout the mass of material which would require some time for this action to be transferred to the surface to be radiated.

Not a physicist here. Pretty much in over my head. If needed, inform me of the errors in my thinking.
DC
 
Last edited:
So let's elaborate on this a little more. The blackbody radiation can clearly be thought of as due to electrons transitioning between energy levels, since it was the inspiration for quantum mechanics. We know that at 0 K, the Fermi-Dirac distribution function looks like a step function with every state occupied below the Fermi energy, and none occupied above. At finite temperature, we get some thermally excited carriers so the Fermi-Dirac smears out and the edge becomes less steep.

Is this the process that causes radiation? Carriers are constantly being excited by radiation that is absorbed, and simultaneously the material radiates as those excited carriers relax back. If the body is hotter than its surroundings, more radiation is being emitted than absorbed, and gradually the population of excited carriers decreases as the temperature of the body falls.

I also know that the Fermi level itself will change with temperature, and I'm wondering if that effect is significant, or for modest temperature changes, can we treat the Fermi level as fixed?

But again, this comes back to the question: if this picture is correct, the material can only emit energies equal to or greater than the band gap, and we know that this is wrong, since they emit mostly in the IR at ambient temperatures.
 
Well, I have learned something more. Solid bodies give off broad-band (continuous) emissions and gases and vaporized material give off characteristic spectral emissions. For someone who worked on flame photometers years ago you would think I would have realized the difference, but I hadn't really thought about it before.

So if LA is still with us after all this I think the simplified answer is: The heat vibrations/oscillations, whatever they might be, in the sand, transfer energy back and forth in the body of the sand but only the surface radiates into "space." Then the warmer material underneath the surface reheats the surface material and then more heat is radiated outward. All this time the atoms in the material are passing heat energy back and forth throughout the material below the surface.
Bottom line; it takes time for the energy to be passed between the atoms and reach the surface where it can be radiated.

I think I have learned enough on this subject for a while.
 
Last edited:
DarioC said:
Well, I have learned something more. Solid bodies give off broad-band (continuous) emissions and gases and vaporized material give off characteristic spectral emissions. For someone who worked on flame photometers years ago you would think I would have realized the difference, but I hadn't really thought about it before.

So if LA is still with us after all this I think the simplified answer is: The heat vibrations/oscillations, whatever they might be, in the sand, transfer energy back and forth in the body of the sand but only the surface radiates into "space." Then the warmer material underneath the surface reheats the surface material and then more heat is radiated outward. All this time the atoms in the material are passing heat energy back and forth throughout the material below the surface.
Bottom line; it takes time for the energy to be passed between the atoms and reach the surface where it can be radiated.

I think I have learned enough on this subject for a while.

I understand, but then why doesn't a liquid do this?
 
Sand, dirt, other solids don't circulate. Liquids do. More complicated. Only thing I know about that is that the hot stuff goes up and the cooler stuff goes down. Does the surface of liquids IN NATURE cool first? Probably. You'll have to look that one up yourself. Or, if you think about the ocean a bit you may be able to understand that the variables are different and what they are.
DC