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How does infa-red heat things up?

  1. Jun 18, 2010 #1
    How does infa-red heat things up?
  2. jcsd
  3. Jun 18, 2010 #2
    'Temperature' of an object is related to the average energy of the particles make it up. The surface particles of the object will interact with the incoming infrared ray (which is an electromagnetic wave) due to the fact that particles are made of charges. This process of heat transfer is called radiation. Then heat will be transfered via conduction to the inner side of the object.
  4. Jun 18, 2010 #3
    The atomic vibrational modes correspond to IR frequencies.
    This coupled with the previous post should answer the question.
  5. Jun 18, 2010 #4

    Andy Resnick

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    The energy is absorbed.
  6. Jun 19, 2010 #5
    Could you elaborate on this? I was wondering why IR frequencies transfer thermal energy/cause random particle motion, which corresponds to temperature increase, and other frequencies (UV, x-ray, radio,...) do not. Is the emission/absorption of thermal radiation as IR frequencies a specific property of all atoms? Are any atoms opaque/transparent to IR frequencies, and if so does this mean that heat can only occur through convection or conduction in the material?
  7. Jun 30, 2010 #6
    I seem to be having an inefficacy obtaining an answer and am wondering if there is an apparent problem with the way I'm phrasing my questions. Am I simply straying too far from the op (should I always start a new thread in order to ask questions), asking too many questions at once, or something else? Any help will be greatly appreciated.
  8. Jun 30, 2010 #7
    EDIT: I just realized this derivation isn't necessary, since it's simply known from a classical harmonic oscillator that frequency is proportional to sqrt(k/m). But I'll leave it here anyway.

    This is adapted from a class I took, and can give you the right numbers. I don't know how intuitive it is though.

    Consider a simple model of a wave in a lattice: a 1-D string of atoms, each with a displacement E perpendicular to the string. The displacement of atom m is Em. Each atom feels a linear restoring force from its two neighbors, with a spring constant C.

    The force on atom m is Fm=C[(Em+1-Em)+(Em-1-Em)]. The acceleration of atom m (supressing the subscript m) is d2E/dt2.
    If you write out the differential equation, and use a harmonic solution E=A*exp[i(kma-wt)], you can get the following result:

    w2=2*C/M [1-cos(ka)], where w is the frequency and M is the mass of the atom.

    Then the frequency is on the order of sqrt(C/M). The mass is on the order of 10 g/mol, or 10^-26 kg. According to a mechanics book I have, the spring constant is on the order of 100 N/m. If you plug these in, you get a frequency of 10^14, or 100 THz, which corresponds to IR wavelengths.

    So this basically says that there are vibrational modes in crystals with similar frequencies to IR, which means the light can couple in to the vibrations very effectively. Of course this isn't the only way light can heat a crystal; absorption of any wavelength will produce heat as the excited electron thermalizes, for example.
    Last edited: Jun 30, 2010
  9. Jun 30, 2010 #8
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