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

Question about colour/light

  1. Jan 11, 2010 #1

    KGC

    User Avatar

    Quick question about colour. As far as I understand it the colour of the object comes from the frequencies of light reflected?

    My question though is when the light absorbs some of the frequencies would this not cause the excitiion and decay causing the material to emit photons of the absorbed colour?

    I am not sure why if all frequencies but blue (for example) are absorbed the object would appear blue?

    Thanks for any help.
     
  2. jcsd
  3. Jan 11, 2010 #2
    Most colours seen in ordinary experience are caused by the partial absorption of white light. The pigments that give colour to most objects absorb certain wavelengths of white light(adding to the internal energyof the object) and reflect or transmit others, producing the colour sensation of the unabsorbed light
     
  4. Jan 11, 2010 #3

    KGC

    User Avatar

    What happens to the absorbed light though, how come it is not re-emitted when the electron goes back down an energy level.

    The wavelengths that are reflected, is this because there is no electron requiring that energy to get up a level?
     
  5. Jan 11, 2010 #4

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    You may want to start by reading the FAQ thread in the General Physics forum. The entry that might be relevant is on the speed of photons through a material. The discussion on the phonon mechanism within the material is relevant here. It tells you that the absorbed energy is directly converted into the vibrational (thermal) energy of the material.

    Zz.
     
  6. Jan 11, 2010 #5
    This only happens when the atom has an energy level that matches the energy level of the photon. If the energylevel does not match the photon energy, it does not go back to the light. The energy instead goes toward increasing the motion of the atoms, which causes the material to heat up
     
  7. Jan 13, 2010 #6

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi ZapperZ! :smile:

    I don't get it. :redface:

    If the "spring" absorbs a photon, it goes into a higher mode (presumably in the form of a roamier electron) …

    why doesn't it then fall to the original mode, and release an identical photon?

    Obviously, it doesn't, and the energy is passed instead to an adjacent "spring" …

    but how does that happen (since they're not actually springs)? :confused:
     
  8. Jan 13, 2010 #7

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    It's not a "coherent" absorption. Think of it as being similar to an absorption of a photon by an indirect band gap.

    The vibration can easily be absorbed by the solid as heat, so there is more than one way (other than photon emission) for that energy to be dissipated. So when you have such a collective excitation whereby the phonon mode is available, the whole solid can absorb that energy.

    Zz.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook