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Prime Colours

  1. Oct 4, 2007 #1
    I've been wondering about this lately. We've most likely all seen the basic example at school of projecting three colours of light at a screen and noticing that the area where the colours overlap turns out to be white. But what I'm wondering is whether there is actually any physics to that. Does the fact that it looks white to us actually have anything to do with properties of those specific wavelengths of light, or is it merely that our eyes have receptors for those three colours?
    From what I understand, true white light is made up of all the different colours of the visible spectrum, so how could three single wavelengths produce the same effect physically?
     
  2. jcsd
  3. Oct 4, 2007 #2

    mgb_phys

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    It's a purely physiological effect.
    The light doesn't actually combine,simply an equal number of red, green and blue photons hits your eye at the same rate.
     
  4. Oct 4, 2007 #3

    Claude Bile

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    We possess three different types of colour receptors (cones), one sensitive in the red, one in the green and the other in the blue. When all three are excited roughly the same amount, our brain interprets this as white.

    Claude.
     
  5. Oct 4, 2007 #4
    If we mix cyan, yellow and majenta, our brain interprets it as white. But why is it that red-blue-green is known as primary colors, and yellow-cyan-majenta known as secondary colors? just conventional?
     
  6. Oct 4, 2007 #5

    mgb_phys

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    cyan yellow and magenta work by each absorbing reg,green and blue - so if you reflect white light from them you get white light back.
    The primary/secondary is just convention.
     
  7. Oct 5, 2007 #6
    Let's say, I put 3 equal amount of red-blue-green colored lights to make a beam of white light. I pass this white light to a prism to cause dispersion. What will be the colors in the dispersed light? red-blue-green, or all visible colors in the spectra?
     
  8. Oct 5, 2007 #7

    mgb_phys

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    Just the red-green-blue of the original lights - if you mix beams of light the individual photons don't affect each other.

    The light from a hot object like the sun or a light bulb contans a range of colours, each photon has a sllightly different colour and the temperature of the object controls how many of each colour is emitted and so the overall colour the object appears.
    For objects that are a few 1000 deg C the amount of red and blue light emitted makes them look white to us because we have evolved under a sun at that temperature.

    You can make a light appear white by emitting a whole range of colours at the correct temperature - like a lightbulb or you can 'trick' the eye into making it think it is seeing white by using controlled amounts of red, green , blue - this is what a white LED or a flourescent tube does.
     
  9. Oct 6, 2007 #8
    I figured as much. Thanks for your replies.
     
  10. Oct 8, 2007 #9

    Claude Bile

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    A more complex example would be a TV screen, which is a whole bunch of green, red and blue dots.

    Claude.
     
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