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Why do things have color?

  1. Feb 9, 2015 #1
    I realize its because of light waves reflecting off of everything, but what gives items their defined color. i.e. what keeps a banana from appearing red and an apple from appearing yellow. Do chemicals have constant way they react to light? why?
     
    Last edited: Feb 9, 2015
  2. jcsd
  3. Feb 9, 2015 #2
    Different receptor cells in your eyes react with different wavelengths of light. What you perceive as color is your brains interpretation of these signals. Interesting to think that what I perceive as red may look nothing like what you perceive as red :-)
     
  4. Feb 9, 2015 #3
    i understand that haha but why do objects maintain their color? As in, why does my eye always perceive an apple as red? :) :) :) :) :) :) :*
     
  5. Feb 9, 2015 #4
    Perhaps because you are avoiding mind altering drugs? :-)

    Different elements absorb/reflect different wavelengths of light (Google: spectral graph), so, assuming the composition of that apple is the same every time, then the colors reflected will be the same. So I guess the answer is, yes, *most* chemicals will have a constant reaction to light.
     
  6. Feb 9, 2015 #5
    Good, good. Now can we go deeper? why do chemical have a specific reaction to light?
     
  7. Feb 9, 2015 #6
    That's a little farther than my understanding, but as a SWAG I imagine is has to do with the size of the atoms vs the wavelength of the light.
     
  8. Feb 9, 2015 #7
    "Light is the result of electrons moving between defined energy levels in an atom, called shells. When something excites an atom, such as a collision with another atom or a chemical electron, an electron may absorb the energy, boosting it up to a higher-level shell. The boost is short-lived, however, and the electron immediately falls back down to the lower level, emitting its extra energy in the form of an electromagnetic energy packet called a photon. The wavelength of the photon depends on the distance of the electron’s fall. Some wavelengths, such as radio waves, are invisible. Photons with wavelengths in the visible spectrum form all the colours that we can see." -Howitworksdaily.com
     
  9. Feb 9, 2015 #8

    DrDu

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    I don't think this explanation is the correct one, here, as it describes rather the emission of light due to heat or chemical reaction, than the interaction of incident light with matter.
    Most coloured substances are not made up of isolated ions but of molecules. In these molecules the electrons have certain accessible orbits with a fixed material dependent energetic separation. Light comes in quanta of discrete energy which is linked to its wavelength or frequency. If light has the correct frequency, so that it's energy corresponds to the difference of the energy of the energy levels of the molecule, it can be absorbed, lifting the electron to a higher orbit.
    The electron most of the time does not emit a photon when it falls back after excitation. Rather, the energy gets converted to heat. That's why black fabric heats up more in the sun than white fabric. The light which does not have the correct frequency to get absorbed, gets reflected or shines trough the substance (as in coloured glass).


    For example, if a substance absorbs light in the red part of the spectrum and we shine white light on it, the green and blue part get reflected and excite the corresponding receptors in the eye producing the impression of cyan.
     
  10. Feb 9, 2015 #9
    The light you see is the light that gets reflected or scattered of an object.
    This is not the same as the light that strikes the object because the object absorbs some light.
    To absorb light, the material of which something is made needs to be able to put the energy of a single light wave (photon) somewhere.
    This means that an electron needs to have a place to go (an 'energy level') that is higher than it's own energy level by EXACTLY the amount of energy in the photon.
    The electron will ignore photons with too much or too little energy.
    If the proper energy for the electron to absorb is, for example, the same as that of a photon of yellow light, and the electron absorbs that photon, then the light that *doesn't* get absorbed will be the color of light *without* yellow - a purplish color you could call 'anti-yellow' - on the opposite side of the Color Wheel.
     
  11. Feb 9, 2015 #10

    DaveC426913

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    This is only the case if the object is subjected to white light. If it were subjected to violet light it would, in fact, appear black.

    It's counter-intuitive but:
    - the colour an object appears to be is determined by the colour of light impinging in it.
    - the colour and object appears to be is only modified by the properties of the object itself.

    A blue object under white light will appear blue.
    A blue object under red light will appear black.

    Any colour object under red light will appear some shade of red, down to and including black.
    (i.e. the colour of the light determines what colour the object appears. The colour of the object in this case cannot determine what colour the object appears, it can only modify its shade of red).
     
  12. Feb 9, 2015 #11
    Thank for the answers everyone. Makes sense to me now.
     
  13. Feb 9, 2015 #12
    Your brain is very good at separating the color of a material from the color of light. You will perceive a red apple as red under many ambient light conditions because you use all sorts of context cues to convert your vision response into a mental image. That's why this optical illusion works. http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html
     
  14. Feb 16, 2015 #13

    Merlin3189

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  15. Feb 16, 2015 #14

    sophiecentaur

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    It's worth while applying a bit of evolution to this - plus practical reality. Never, in our past history, have we ever encountered illuminants with anything other than black body spectrum, modified by some atmospheric effects. So stark statements about what things will look like under narrow band illumination, whilst true, may not be entirely relevant. The eye / brain copes very well with assessing the 'colour' of objects under pretty much all natural lighting conditions (except in very low light where the rods are the only ones operating). The principle of 'integrating to grey' seems to be what our colour sense works on. The eye works on the principle that most scenes present a similar mix of colours (averaging out to grey). As with colour cameras on 'Auto Colour Balance', the gains of the three colour channels are adjusted to give a final colour mix that is 'grey'. An orange shirt, seen on top of a mountain at midday or in a valley under a 'red sky' will be assessed as very much the same colour, after the brain has processed it. Colour memory can vary a lot from person to person, of course, and we can't rely on it to match an orange shirt, bought one day, with an orange skirt, both on another day, in a different shop. LEDs and CFL's with their crazy spectra, also add confusion to colour sense. It does a pretty good job, overall.
    Colour matching with dyes and pigments is really fraught and CMY mixing can never be relied on to give a consistent colour for well know Brand Colours. Wherever possible, printing uses specially mixed Spot Colours so that colours on paper, cardboard, paint and fabrics will be recognised as the same by the public without confusion. It's a serious problem to make a woollen top match a cotton skirt - in the shop, out in the street and at home.
     
  16. Feb 16, 2015 #15

    DaveC426913

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    I walked out of a bowling alley late one night, into the parking lot, which was lit by strongly orange sodium lamps. It took me several tries to find my (orange) Toyota. My brain unconsciously filtered out the colour cast, leaving my car appearing quite distinctly silver (colourless), and I kept walking right past it.
     
  17. Feb 16, 2015 #16

    sophiecentaur

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    A victim of modern technology. Ancient Man just didn't learn about the Sodium Yellow lines. :wink:
     
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