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Homework Help: UV vs IR in heating objects

  1. Jan 1, 2014 #1
    I received a handout at university that I believe could be erroneous.

    It states. "IR radiation has a higher potential to warm objects than UV light. Does IR therefore have a higher energy level?"

    It refers to E = hf

    Note it mentions 'UV light' but 'IR radiation', not written by a physicist, this text. Background is biology spectroscopy

    Now I know at ordinary temperatures almost all objects emit radiation in the IR range. And therefore IR is sometimes wrongly called 'heat'. It seems some of my teachers believe this as well and are mistaken.

    Or does this refer to the different nature of IR vibrating molecule bonds and UV causing electron transitions. Since this is QM I can't use logic or common sense (or do the math, lol) and googling and searching in this place doesn't make me confident enough.

    It ought to be the case that all forms of radiation, if they contain equal energy (meaning up intensity of radiation with longer wavelenghts) that they heat up objects equally. This because molecules absorb photons of all energy levels. No preference for IR photons.

    I can't necessarily nail down higher electron states with more kinetic energy in the molecule with higher temperatures to tell my teachers they are wrong.
  2. jcsd
  3. Jan 2, 2014 #2

    Simon Bridge

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    Start by answering the question.
    Never mind, for now, what you think your teachers believe is the correct answer - just do your best to supply the correct answer.

    (Note: the word "level" in the question is misplaced. It properly belongs to the irradiated object not the light itself.)

    Why should that be the case? If all objects really absorb radiation of all wavelengths equally, then where do absorption spectra come from? How come some objects, like glass, are transparent to some wavelengths?
  4. Jan 2, 2014 #3
    The electromagnetic energy content of a photon is inversely related to that photon's wavelength and directly related to its frequency. Therefore the answer to the question is, "No."

    As to your last paragraph, the temperature of a gas is a function of its kinetic energy of translation. It is independent of its internal energy.
    Last edited: Jan 2, 2014
  5. Jan 2, 2014 #4
    Never mind the actual question. It's trivial anyway.

    They don't. It depends on the absorption of the object. But that is not a given variable in this case and therefore we can ignore it.
    Or is it true that almost all objects absorb IR much more readily because that is what they would emit?

    Is the question being dodged? If you don't know the answer, please don't reply.
    I see that second person hints at the ideal gas law. Seems people with less knowledge than me are replying...
  6. Jan 2, 2014 #5

    Simon Bridge

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    Is there some reason you don't want to answer then?
    Never mind - you were provided with the answer in post #3.

    ... so when something is not written down it is not important?
    Is that a safe assumption?

    So you are saying that given equal absorbance for the wavelengths involved, and equal energy carries in the radiation, the amount of heating should be the same ... but that assumes that all the EM energy goes into heat.
    Are there other channels for the energy to go - for example, could it go into ionizing some atoms/molecules in the material?
    Would IR or UV wavelengths be more likely to ionize the sample?

    Anyway - the question, as written, is unspecific about what material is doing the absorbing - therefore the absorbance of the material is a variable. In general, not everything that you need to solve a problem will be presented to you in the problem statement.

    No - most objects more readily absorb infra red. They can, and do, absorb other wavelengths quite readily - see their absorption spectra - and do not always radiate the same wavelengths that get absorbed - how do objects get their colors?

    Not at all, klimatos was referring to the kinetic theory of gasses ... which can be used to derive a number of gas laws depending on how one sets up the particle model. The hint is in relation to the "last paragraph" where you complain about having trouble relating energy and temperature.

    Please read the rules for the forum - it is unusual for anyone here to just hand out the answers.
    The idea is that you learn more if you figure out the answers yourself.
    Mind you, it would help if you had actually posted a question that you wanted to answered.
    Instead you made a bunch of statements resulting from a question you thought was "trivial".
  7. Jan 2, 2014 #6


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    Please check your PMs. Insults are not tolerated here on the PF. Simon is trying too help you figure out the answer, instead of spoonfeeding it to you.
  8. Jan 2, 2014 #7


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    Thread closed.
    Last edited: Mar 14, 2014
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