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Thermal Radiation v Absorption of Light

  1. Jun 23, 2011 #1
    A layman with an interest in quantum mechanics here.

    Just want to check I understand something...

    Is the phenomenon of electromagnetic waves being emitted by an atom due to it being hot (black body radiation) different to that of such waves being emitted as a result of absorbing light (from the sun for example.)

    As such, is this correct...

    If I was in a room at night (no sunlight) with no lights on, my hand would emit low frequency radiation (infra red light) due to my body heating up the atoms in my skin. If I then shone a torch on my hand, the atoms in my skin would absorb the torch light and emit visible light (as well as the infra red light) making up the pinky colour of my hand.

    If this is the case, when we look at the spectra of a distant object how do we know whether it is emitting light due to heat or due to reflecting light. For example, the atmosphere of venus reflects the sun's lights.

    My head hurts. Please help.

  2. jcsd
  3. Jun 23, 2011 #2


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    Science Advisor

    Ok ... I think one of the issues is that there is a difference between emitting light and reflecting light that is not carried through in your analysis. The emission spectrum of a black body is, as you say, determined by its temperature. An ideal black-body doesn't reflect light .. it absorbs it. The energy from the absorption gets thermalized among the modes of the black-box, before eventually getting re-emitted as part of the normal thermal spectrum.

    Non-ideal black-bodies, like your hand, or Venus, have black-body-like properties (i.e. absorption and thermal spectrum), but also reflect light. Reflection of light is not really absorption and re-emission .. the energy from the reflected light never becomes thermalized, but simply "bounces off". To complicated matters, there are two kinds of reflection .. specular reflection ("angle of incidence = angle of reflection"), and diffuse reflection, which is essentially a scattering phenomenon whereby incident light is scattered off the surface of an object in all directions. Most of the time we "see" the light from diffuse reflection.

    In any case, the principal qualitative difference in the context of your question is that reflected light represents the spectrum of the incident light (with any absorbed components subtracted out), rather than the thermal spectrum of the reflecting body.

    In the case of far away objects, what we assume about them depends on what we know about their distance. If something on the scale of a star (or planet) is hot enough to emit in the visible spectrum, then the light will be many many orders of magnitude more intense than reflected light from an object of the same size. So, the only bodies we see by reflected light are fairly large objects in our own solar system, and the light that is reflected from them has (for the most part) the same visible spectrum as our sun. These objects appear large enough that they are clearly not point sources, when observed at appropriate magnification. Outside our solar system, we can only "see" (meaning detect through observation in the visible part of the spectrum) objects that emit (lots of) light due to their thermal spectrum, i.e. stars. The fact that stars are generally much larger than planets also helps to increase the intensity of the visible light they emit, although they are still far enough away that they appear to be point sources of light.

    Ok .. I see that I am rambling a bit .. I'll stop now and see if that helps answer your question.
  4. Jun 23, 2011 #3

    So actually if I shine a torch on my hand in a dark room the following happens:

    1. Thermal radiation continues (infra red) as my body is still heating the atoms on my skin.

    2. Some of the torches light frequencies may be absorbed by the atoms in my skin which adds to the non-visible thermal radiation.

    3. The rest of the torch's light is reflected which creates the pinky colour.

    So the pink colour depends more on what I shine on the skin rather than what atoms are on my skin. So if was to shine my red astronomy torch on my skin I would not see pink.

    So examining the spectra of Venus allows us to identify the bits of sun light that are not reflected as some of the sun's wavelengths are used by Venus' clouds for non-visible thermal radiation. And as Venus is too cool to emit visible light we know that all the visible light is reflected and not emitted.


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