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What exactly is color?

  1. Nov 20, 2007 #1
    I don't think i clearly understand what color is on the molecular level... white light strikes a surface, and some of it gets reflected, and some gets absorbed, and you perceive the color based on what light gets reflected. That is ok, but what property of the material or the atoms that make it up exactly determines if light of frequency f gets reflected or absorbed?

    Also, human eye has 4 rods, 3 for R G B, and 1 that apparently senses brightness. But can't you specify exactly every shade of color only using the 3 RGB components? Why do you need the 4th rod to pinpoint the color?

    The second question is more of a biology question for fun, but mainly im wondering about the first one.

  2. jcsd
  3. Nov 20, 2007 #2


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    This is not really my area, but I thought it was something like this.
    You should imagine that the electrons in a molecule have different orbitals, with different energies. Normally they are in the lowest orbital, but if you add enough energy to bridge the gap between two of them, they can go to a higher one (we call this an excited state). One way to add this energy would be to shoot in a photon with enough energy (which depends on the frequency through [itex]E = \hbar \nu[/itex]). Since the excited state is not a natural state, eventually the electron will fall back to its ground state and emit the energy difference between the excited state it was in and the ground state it's going to. This energy can be released in the form of a photon, which we perceive as reflected light. Or the energy could be lost (e.g. dissipated to the surroundings, used to excite another electron, etc) -- in fact the incoming photon may not excite an electron at all because it just doesn't have enough energy, and just lose its energy going through the molecule. Different atoms are sensitive to different energies (the energy needed to excite an electron differs, hence the frequency of the light you need to send in) and that's why different materials have different colors.
  4. Nov 20, 2007 #3
    You're familiar with guitar strings (or the air in a hollow bottle) resonating with particular frequencies of sound. Comparably, the the electrons in particular molecules resonate with particular frequencies of electric field waves.

    Regards rods, you've forgotten cones... read http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html" [Broken]say.
    Last edited by a moderator: May 3, 2017
  5. Nov 20, 2007 #4

    Claude Bile

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    You are looking at two separate issues here - the electromagnetic radiation itself (sometimes called radiometry), and the human response to that radiation (sometimes called photometry). Let's start with the radiometry first.

    We can classify objects into two broad categories, emitters and non-emitters. The colour of an emitter depends on the spectrum of the electromagnetic radiation it emits (i.e. a "red" light is one that emits red frequencies). Non-emitters on the other hand have their colour characterised by their absorption spectrum. Frequencies that are NOT absorbed contribute toward the colour of that object, for example a "green" leaf absorbs red and blue but not green frequencies.

    The colour of an emitter can depend on several things, depending on the nature of the emitter. Atomic sources, such as gas lamps and lasers have colours that depend upon atomic transitions. For example a mercury lamp will emit frequencies that correspond to allowed energy transitions (via E=hf). On the other hand, LEDs depend on the semiconductor bandgap, thermal (blackbody) sources depend on temperature etc. There is no universal parameter that determines the colour of an emitter.

    The colour of a non-emitter depends on the what we call the band structure of the object, which is basically how the various energy bands of the solid are arranged. This is distinct from the atomic transitions mentioned above as they depend on the collective properties of a whole bunch of atoms and molecules, rather than individual atoms.

    Okay, on to the photometry - you have 4 kinds of photodetectors in your eyes, rods and 3 types of cones, each sensitive to a different primary colour (red/blue/green). Rods are useful because the are more sensitive to light and are predominant in low-light levels (that's why it's hard to see colour when it's dark). Rods also comprise the bulk of our peripheral vision.

  6. Nov 20, 2007 #5
    The 3 types of cones respond to coloured light, and I won't try to expand on Claude's description, it seems clear enough. As with any detector it perforce has a higher threshold to ensure that when it triggers it gives an accurate or representative sample. You wouldn't want a green receptor going off for red by accident as it would throw off the point of the detector in the first place.

    Rods on the other hand, primarily in the periphery of the retina handle low light conditions and respond far less to colour and more to light intensity.

    Basically one handles colour and the other greyscale. Two seperate systems that work together to give your vision a fallback position between full colour vision and blindness.
  7. Nov 21, 2007 #6

    Andrew Mason

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    Zapperz has https://www.physicsforums.com/showpost.php?p=899393&postcount=4" [Broken]:

    Last edited by a moderator: May 3, 2017
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