Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Light: Reflection, Re-emission, or both?

  1. Dec 19, 2003 #1
    In basic science class, when talking about light and the colors of objects, people will say that the pigments absorb some light, but the light that represents the color you see is reflected.

    Is light actually reflected (the same photons/waves still exist and are now travelling in a different direction as a result of the interaction with the material), or are they merely absorbed and new light is emitted by the material? Maybe a little of both?
  2. jcsd
  3. Dec 19, 2003 #2

    Chi Meson

    User Avatar
    Science Advisor
    Homework Helper

    Take a smooth, flat, black object, something like obsidian or polished marble, or a very glossy painted surface (I'm looking at a painted metal disk). You will notice that, to some extent, there will be some specular reflection (like a mirror)that will still have the color of the thing that is reflected. Yet the thing is still "black" which means it's supposed to absorb all light that hits it.

    This is some evidence that indicates two kinds of reflections going on. The specular reflection is not an "electron energy-level" absorbtion reemission; instead it is reflection due to the outermost electrons which are loosely bound to the nucleus and can reflect any wavelength of light.

    THe color is still black because most of the light that falls on the surface is still absorbed. IF you are looking at a blue object, most of the blue light that you see is being absorbed and reemiited back at you, but some of all wavelengths of light will still be reflected by those outer electrons. So depending on the surface material, a bit of both will occur.

    I don't think that you can say for either type of reflection it is the "same photon" in the sense that it's the same tennis ball that you hit back to your opponent. But also, you can't say that it's a different photon either; if it has exactly the same wavelength, then it is the same package of energy coming out as the package of energy that went in. A philosophical point, I think.
  4. Dec 19, 2003 #3
    Thanks for the response.

    However, I don't quite get the last response, and it's related to what I was wondering about. It it the actual same physical entity that is now moving in a different direction, or is it a replica?
  5. Dec 20, 2003 #4

    Chi Meson

    User Avatar
    Science Advisor
    Homework Helper

    Tricky question. A photon essentially replicates itself continuously as it propagates through space (through electromagnetic induction). We have a problem in defining what actually is the entitiy that is a photon. At the root of it all, the photon is a package of energy that exists as an electric and magnetic field "undulation." IT behaves sometimes like a particle, but not enough to liken it to a tennis ball.

    WHen a photon is absorbed, the energy is momentarily contained by an excited electron. At this point there is no photon. THen the energy is released by the electron and with this de-excitation, a photon is created. So since there was no photon at some point, you could argue that a different photon was created.

    Various things could happen that would make some of the properties of the photon different : it's polarization could shift, its direction of propagation could change (refraction or scattering), and these could be reasons to call it a different photon.

    I believe that reflection from a metallic surface would also cause there to be an instant where there would be no photon, so this form would be in the same boat. THe big difference is that metallic reflection does not cause polarization of reflected light.

    But then, I just repeat: if it's teh same energy package, who's to say it's not the same photon?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook