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Light- Matter Interactions

  1. Mar 28, 2014 #1
    Could someone please clear this up for me. We have,

    Reflection (metal, mirror)
    Reflection (colour)
    Absorption (heat)
    Emission (heat)
    Transmission (glass)

    Sry but I use photoelectric effect to reference electrons absorbing photons of specific energies to be excited to higher energy levels, I don't know if it's strictly the term I should use but you get my point...

    Firstly, glass.

    Transmission: glass is able to transmit visible light because it's electrons cannot be elevated into a higher energy level by these frequencies. This I know is true.

    Emission: A spectrum of frequencies are emitted by a body depending on their temperature. Hotter bodies spectra centred around higher frequencies. Doesn't involve electron energy levels. I know this to be true.

    Absorption: I can only assume that this doesn't involve electron energy levels either (photoelectric effect), because a black t shirt absorbs all wavelengths of visible light...

    Reflection: green t shirts absorb all light except that of green wavelength that they reflect. Is this the electron energy level mechanism? If so, how can white t shirts reflect all visible wavelengths in this way... Same intuition as black t shirts, it doesn't make sense to call it as photoelectric effect type interaction.

    So the problem... Glass is transmissive to visible because it doesn't interact via photoelectric, but then how do you explain heat absorption and reflection as photoelectric effect as black and white t shirts would require many different electron ground state energies to absorb/remit the whole visible spectrum in this way.

    Extra bit, metal mirror: Specular reflection as the surface is smooth vs diffuse reflection in the rough surface of the white t shirt. Delocalised electrons reflect light of all visible wavelengths, apparently I have read that is not photoelectric effect either, it is to do with the fact that the e are delocalised that they can interact with the em field and reflect the energy, but I don't know how this done either.

    So there it is, I was told at A level that lights interaction with atoms was mostly photoelectric e=hug photons but then the above doesn't suggest that it is at all. I know high frequency incident light is however, ionising radiation etc. I wasn't taught anything about how it's electromagnetic field interacts, heat is absorbed emitted and reflection mechanism, only the standard angles of reflection and snells law and diffraction etc

    Please clarify/suggest a book that details all this.

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  2. jcsd
  3. Mar 28, 2014 #2

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  4. Mar 29, 2014 #3


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    The photo-electric effect is not a good place to start for optics.

    Nor is absorption the only possible interaction between light and matter.

    Here is an elementary introduction: http://celsus.as.arizona.edu/~cmeakin/pima2010/LightAndMatter.pdf
    It will provide you with a better starting point.

    This is a more advanced presentation, using many of the same slides:

    Here is a module on light, with a self-test:

    Undergraduate physics texts which cover electromagnetic field theory, or an optics text will go into this material very thoroughly. For example: Griffith's "Introduction to Electromagnetism"; Hecht "Optics".

    Or the Feynman "Lectures on Physics", volume I.

    Note: these forums have discussed other issues with the content of the A level physics courses. They seem to be in need of an update!
  5. Mar 29, 2014 #4
    Thanks for this, this is great. Couple of queries though...

    There are two mechanisms for creating light. Black body thermal continuous spectrum, caused by inter atomic/ intermolecular collisions/ charged particle interactions. Nothing to do with electron energy levels.

    And emission/absorption lines, which are exactly that.

    So firstly to clarify, everything should have both. A light bulb will have a black body and tungsten (absorption or emission?) lines, and hot hydrogen gas will have a black body and hydrogen emission lines as shown in the slideshow, (added emissions from molecular collisions/vibrations exciting electrons then dropping down).

    So am I right in saying the absorption process must be through electron energy level resonance, ie there is no direct absorption counterpart to black body emission?

    Then there is slide 47 of the first document. There is a spectra associated with molecular rotational/vibrational emission ( hydrogen gas shown) Is this actually black body emission but in practise rather than in theory?

    Finally a red object appears red as when exposed to a spectrum it absorbs the other primary colours as heat and reflects/ scatters red. How does the reflection/scattering process occur? As a reflected wave or as remitted photons?

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  6. Mar 29, 2014 #5
    Read up on slide 47, it's relatively weak and due to the electric dipole of the molecule rotating or vibrating sinusoidally and so emitting light, so I get it.

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  7. Mar 30, 2014 #6


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    You must also consider scattering ... which is independent of energy levels. For example, transparent objects transmit light via coherent forward scattering - the Huyghens wavelet model gives the idea.

    Metals strongly absorb at all frequencies - but the induced electron motions from the incoming light wave generates an outgoing replica; depending on the metal and the wavelength this process is more or less efficient ... 50 to 90% for common merals, the modern techniques for changing the surface can improve this.
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