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

Seeing colors: photons vs waves

  1. Apr 9, 2013 #1
    As an atmospheric physics major I am familiar with electromagnetic radiation in the atmosphere and what dictates what wavelength objects will emit at. When observing radiation in the atmosphere it is always thought of as a wave, whether it be longwave or shortwave. Recently though I have been introduced to the quantum world and I am having trouble translating between the wave model and the particle model of light.

    I understand that the sun, that has a temp of 6000k, has its peak emission in the visible spectrum. When that light makes it to Earth, and hits an object, that light will be absorbed and what is not absorbed will be reflected back and we will see that color. I am having trouble understanding how this works when thinking of photons. How do we see colors of an object when thinking about photons?

    Recently I watched video on why glass is transparent. It said that the electrons in glass were arranged in such a way that when they encountered photons there was not enough energy for them to reach a higher energy level. This made me think of why absorbing that photon and reaching a higher energy level is necessary to not being transparent. It made it seem like that in order to not be transparent photons had to be absorbed so electrons could emit a photon of that color.

    When thinking about photons, do we see colors because photons are being reflected back like a wave does? Or do we see electrons emitting a certain color of photon as it lowers energy levels? For instance, I have a green wall with a window. Are photons passing right through the glass but hitting my wall, exciting the electrons up a level then as they go to a lower level a green photon is emitted or do the photons hit my wall and jsut bounce back?
  2. jcsd
  3. Apr 10, 2013 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF;
    Color vision (seeing colors) is a biological artifact to do with the way our eyes work.

    Put crudely - your eyes are measuring how hard the photons strike the photosensitive bits ... that tells it the incoming momentum and energy and so the wavelength and thus the color.

    Different objects have different colors because they interact with photons differently - the basic interaction is between a photon and an electron, and the situation of the electron controls what sorts of things can happen and what the probabilities are. What you get scattering off is the weighted sum of all the possibilities.

    To your main qestion:
    It is all of the above.

    The basic interaction is a photon is absorbed by an electron - and the electron wants to get rid of that energy somehow. It can do this by releasing a photon of the same wavelength, by transferring energy to the bulk material is is bound to, or by releasing a number of different photons. All these contribute to the overall color.

    iirc the dominant effect for diffuse color is absorbtion - where a material favorably absorbs particular wavelengths of photons and dissipating the energy through the bulk material - heating it up. The primary pigments are derived from a single primary of light being absorbed - i.e. cyan surfaces favorably absorb red photons. This is why the primary colors for paint and for light are different and why, when you mix blue and yellow light you don't get green light.
  4. Apr 11, 2013 #3
    I am also trying to really understand these basic properties of light. This question was one I was also thinking about. If light passes through glass, is it being emitted as photons on the oppisite side by the atoms? Or is it more like a gamma ray passing through with very low chance of interaction?
  5. Apr 13, 2013 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    All light is photons... does not matter what emits it.
    Gamma rays are a kind of light ... so it is exactly like gamma rays going through.

    Light encountering glass may be reflected at the interface, or refracted.
    Refracted light may be transmitted through or absorbed. Absorbed light may be re-emitted.
    But it is sensible to think of the light emerging from one side of a sheet of glass as being the same light that entered the other side.
  6. Apr 16, 2013 #5


    User Avatar
    Science Advisor

    Most of the visible light passes through glass without being absorbed. If a photon is absorbed by a material and re-emitted, then the new photon goes off in a random direction. This scatters the light. In quality glass, only a small fraction of light is scattered.
  7. Apr 16, 2013 #6
    Do0 a search here in the forums...several good discussions on your issues....
    If you are not familiar with the double slit experiment check it out..carefully.

    There light always behaves like a wave but is detected as particles [photons]. Fields are an artificial construct...one which works remarkably well, but what we detect are local photons.

    I like to save insightful comments..here are some I like:

    QUOTE]How big is a photon

    The size of a photon depends on its environment……..{no!!} You are still talking about the probability distribution, not the location of a particle. If you insist on talking about EM field in terms of particles, photon is point-like.

    Perhaps what he was getting at is that when detected, the photon has size that interacts with the detector with point-like properties, but while propagating before detection it has size that is described with wavelike properties distributed over the meter distance. IOW, I suppose he was trying to emphasize wave particle duality and that both are legitimate descriptions of the real world, with an equal claim to the concept of "size."

    Carlo Rovelli:
    This is a pretty advanced explanation but you can discern how field models are developed:

    presented in Weinberg's "The quantum theory of fields" vol.1....
    I have virtually no idea what a Fock space is other than some kind of linear model based on Hilbert space. Everybody seems to build linear models..even Einstein....

    Marcus' comment:

    Very often, the model we use is an approximation based on the physical conditions..often high energy and low energy approximations....
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook