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Mixed light and pure light?

  1. Jan 27, 2010 #1
    Mixed light and "pure" light?

    We all know from our young days that mixing paints give you different colors, like yellow and blue gets green.
    What i wondered about is what kind of light will it be?

    If you shine blue and yellow light on something, you will see green. There is also not much difference from natural green emitted by an LED for example. And the screen you look at makes yellow by mixing red and green.

    Is it a perception thing or is there some frequency merge occuring?
    What are the differences?

    For a photoeffect made by shining mixed green light on proper metal will it be for green frequency or for blue?

    What will a coloblind person see in the mix?
  2. jcsd
  3. Jan 27, 2010 #2


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    Re: Mixed light and "pure" light?

    This is not necessarily correct. You will see whatever frequencies of light are reflected from the illuminated substance. If it reflects both blue and yellow light comparably, (e.g. a white surface) then it will indeed look green. But if the substances absorbs one of the illuminating colors and reflects the other, then you will only see the reflected color. For example, a purple pigment absorbs yellow light, but reflects blue, so it would look blue under the conditions you describe (assuming no other sources of light are illuminating it).

    Try a google search on "complementary colors of light" for more info on this ...
  4. Jan 27, 2010 #3

    Claude Bile

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    Re: Mixed light and "pure" light?

    Mixing paints is commonly described as "subtractive" colour mixing. You start with white, and then filter out different wavelengths (colours) using various pigments. You get green paint when you mix yellow and blue (cyan) because the yellow and blue pigments, collectively absorb all colours except for green. This process depends quite critically that you begin with white light, if you view the paint under, say red light only, you will not perceive green.

    With LEDs however, the colour mixing is additive. You start with black (no colours) and then add wavelengths using your light sources. If you mix yellow and blue light, you will perceive green because of the way eyes perceive colour. Eyes contain three different colour receptors (cones) that are sensitive to red, green andblue wavelengths; however the colour range over which each of these receptors detect light overlap quite a bit. This means that some blue light will also be detected by the green receptors, while yellow light will excite both green and red receptors. The end result is that the green receptors can detect a sufficient amount of light to perceive blue + yellow light as being green.

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