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Infra-red is always considerd responsible for radiant heating

  1. Aug 27, 2003 #1
    It seems to me that infra-red is always considerd responsible for radiant heating. Why is this so, what makes infra-red so special when it comes to heating things? I would have thought that higher frequncies would tend to 'feel' hotter, but this doesnt seem to be true.
  2. jcsd
  3. Aug 27, 2003 #2
    I think the reason is that IR frequencies are more easily absorbed than higher frequencies, causing the material molecules to vibrate faster. However, this is not valid for all materials. I think this deppends on the type of material.

    Have you been at the beach in a cloudy day? You don't feel the same heat like when there are not couds because the some of the incoming IR radiation is absorbed by the clouds, and your water molecules (you are almost 70% water) do not absorb the same amount of radiation.
  4. Aug 27, 2003 #3
    When an object is heated emits radiation over an spectrum of frequencies, depending of the temperature. At ambience temperature, practically all the emitted radiation is infrared.

    If you continue heating the object the spectrum of frequencies is shifted towards higher frequencies (towards the visible). That’s the reason why when you heats a metal, firstly you don’t see anything, then you begin to see it red, clear red, and white before it melts.

    I have found this table at Internet.
    The contribution of the different frequencies to the intensity of the radiation emitted is:

    Temperature (K) % Infrared %Visible %UV
    1000 99.999 7.367·10-4 3.258·10-11
    2000 98.593 1.406 7.400·10-4
    3000 88.393 11.476 0.131
    4000 71.776 26.817 1.407
    5000 55.705 39.166 5.129

    Jain P. IR, visible and UV components in the spectral distribution of blackbody radiation. Phys. Educ. 31 pp. 149-155 (1996).
  5. Aug 27, 2003 #4
    mind posting a link to that site?
  6. Aug 27, 2003 #5
  7. Aug 27, 2003 #6


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    Re: Infra-red

    The reason is the temperature of things that do radiant heating. Everything from a warm sidewalk to an electric heater does most of its radiating in the ir part of the spectrum.
  8. Aug 28, 2003 #7

    Chi Meson

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    Another thing to consider is that visible light is much shorter in wavelength than the bulk of the IR range. The wavelength of the photon influences how it will react to certain things.

    Even though visible light also "heats" things up, a larger portion of incident energy in this region is re-radiated.

    In UV and X-ray photons, the energy is so great that it wouldn't make your skin feel hot, instead it would just destroy your skin. No time to go "OW."
  9. Aug 31, 2003 #8
    Re: Infra-red

    Radiation (light) in classical electrodynamics can reflect off, transmit through, or be absorbed by a material. Now, different materials have different characteristics of what type of radiation they absorb, reflect, and transmit the most and least. But just because an object can absorb lots of radiation like visible light, doesn't mean that the object will have a measureable change in temperature, so this must not be the important issue.

    The macroscopic quantity you call temperature is a measure of the average kinetic energy of the microsopic consituents of a material. Therefore, to increase the temperature of a material due to radiation, the constituents must gain kinetic energy so that, on average, it is higher than before (e.g. the ions in a metal jiggle around their equilibrium positions).
    Now, materials around us tend to be made from medium weight atoms and molecules that vibrate about some equilibrium position (that they would sit at if they weren't moving). As photons in a light ray hit these atoms and molecules, they can excite the atoms/molecules in different ways depending on the energy of the photons (and therefore the frequency of the light via E=hf).
    If the energy of the photons is high enough, they can ionize the atoms/molecules (ejecting electrons from them), or if it is even higher, they could eject neutrons in the nucleus from heavier atoms like Uranium (then in appropriate conditions we could get a weapon of mass destruction or a useful source of boiling water to generate electricity). However, lower energy photons can excite vibrational modes of the atoms/molecules themselves, or translational modes if the atoms are free to slide around like in a liquid. At even lower energies, the photons can excite certain rotational modes of certain molecules in e.g. water...then we have microwave heating.
    Now, for the medium heavy atoms/molecules we are talking about, like in our skin e.g., the photons that are to excite the biggest vibrational modes of the molecules, and therefore result in a higher average kinetic energy (and therefore higher temperature by our definition) must have a particular range of energy. Therefore, the light we use must have a certain range of frequency (via E=hf). It turns out that this is the infrared part of the radiation spectrum.
  10. Aug 31, 2003 #9
    first of all... what to you mean by modes?

    you say photons of different energies do different things (ionize, vibrate, split nuclie) to different molecules. That doesn't make sense to me. Seems to me that higher energy photons(X/gamma rays) might be able to ionize molecules, but I would think they would cause the lower order effects sush as just vibrating the molecule thus rais the temp. before they ionized or split nuclie. To sum it up...I don't understand why they wouldn't just do the same things IR does, but do it to a greater extent.
  11. Sep 1, 2003 #10
    Don't leave me hanging javier!
    I was for the most part content with this thread, but you have sparked my interest again.
  12. Sep 1, 2003 #11
    A "mode" comes from "normal modes" which is the term given to, e.g., the discrete set of standing waves you can set up on a string.
    Consider a classical harmonic oscillator (for concreteness, suppose you hold a mass on a spring hanging vertically). You can, for our purposes, model an ion in a metal lattice like this. You can drive the mass at different frequencies by shaking the spring up and down at those frequencies. If you shake too fast, the mass does not respond to much in its amplitude of oscillation. If you shake up and downtoo slow: the same. But, there is a particular range of frequencies where the mass responds significantly, and in fact there is a particular frequency (the "natural frequency") where you will get the greatest response in amplitude from the mass (resonance). So if you have materials made of mediumly heavy atoms, like the ones around us, then there is a certain set of frequencies of light that will give the atoms a significant oscillation amplitudes. If the atoms, pictured as little harmonic oscillators, have larger amplitudes of oscillation, they must have more kinetic energy on average than before, and therefore the material will have a higher temperature.
    Now, a harmonic oscillator in quantum mechanics can oscillate with a discrete set of frequencies, it is not arbitrary. This set of frequencies are called the "modes" in analogy with standing waves on a string. In general in quantum mechanics, whenever things rotate or oscillate in a discrete set of possible ways, we call the set the "modes" of rotation or oscillation.
    Let's go to molecules to give examples that I gave before. In quantum mechanics, there are discrete modes of rotation, meaning a molecule can rotate only with particular frequencies (a discrete set of them), and therefore particular energies. To excite these "modes of rotation", you need photons with *those* particular energies...it won't work if you send in photons with too much or too little energy (this is quantum mechanics we're dealing with now, not classical mechanics of rotating balls).
    Now, the same idea is true for the electronic structure of an atom. If you send photons toward an atom with arbitrary energies, nothing in general will happen. But the electron can have a discrete set of energies (these are the energy levels discussed in basic courses for things like Hydrogen). Only when a photon has the right energy to excite an electron from some low level (like the ground state) to a particular higher energy level will it do so. Otherwise, the photon is not "absorbed by the atom".
    Again, a similar thing is true of the nuclei of atoms. The nuclei can be excited to higher energy levels, while still in a bound state with each other. But the energies of the photons required to do this *must* be of certain energies that happen to be higher than to excite electrons in the atom. Loosely, we can say this is due to the stronger interaction of the nuclei with each other as compared to the electromagnetic interactions between the electron and the nucleus.
    The key to all of these facts is that these particles obey the rules of quantum, not classical, mechanics. As a result, only certain energies are available for a molecule to vibrate, rotate, and for the electronic structure of the molecule. Therefore, to "excite these modes", we have to provide a particular, not arbitrary, amount of energy. Photons are quanta that have particular amounts of energy and so only certain frequencies of light can excite these modes.
  13. Sep 1, 2003 #12


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    Thank you for your execellent posts on this (and other) topics.
  14. Sep 1, 2003 #13
    Yeah Jav, that's great.
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