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Optics and Colors

  1. Jun 26, 2006 #1
    Why is it that if I were to mix a blue and yellow liquid together, my eyes see green? Why don't I see a mixture of blue and yellow dots?
  2. jcsd
  3. Jun 26, 2006 #2


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    You can if the liquids don't mix properly. Basically, it comes down to the resolving power of the eye. Just as you can't make out the individual pixels in a photograph, even incompletely mixed colours will appear uniform unless magnified significantly. If, on the other hand, a chemical reaction rather than a physical mixing occurs, the new colour is even down to the molecular level (eg: litmus paper).
  4. Jun 26, 2006 #3


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    Staff: Mentor

    Perhaps you would if you could see the individual molecules...
  5. Jun 26, 2006 #4
    But why do I see green? Why not some other colors like purple?

    Further, we see something blue, because that object absorbs all wavelength except blue wavelength. If I were to mix blue, red and green liquid together, why don't I get a white liquid? But why blue, red and green light mixed, gives you white light?
  6. Jun 26, 2006 #5


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    There is a difference between addative and subtractive colours. In your monitor, for example, red, green and blue are the produced colours. If all of them are on at once, you get white. In 4-colour process printing, which is essentially what your desktop printer does, the pigments are magenta, cyan, yellow, and an auxilliary black. M+C+Y= black, but it's more of a muddy brown so a true black cartridge is usually added.
  7. Jun 27, 2006 #6


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    It comes down to the relative stimulation of the colour receptors in the eye. There are only three types of receptors: red, green and blue. They are not perfect and are stimulated by a range of frequencies (a bell curve).

    Look at the attached diagram. Read top to bottom.

    You see, the green receptors in your eyes are stimulated by BOTH the blue light AND by the yellow light, whereas the red receptors and blue receptors are less stimulated. Upshot: you see green.

    Danger hit it in the head. Pigments subtract colours; light sources add colours.

    All pigments mixed together manage to absorb all wavelengths, leaving nothing (black). All light mixed together emit all wavelengths, leaving white.
    Last edited: Jul 20, 2007
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