On Sun, Mar 30, 2003 at 05:19:35PM +1000, Fay wrote:
> Hi Daniel,
> I don't think that either set of colours has to do with how our eyes work.

Actually, it does.
- Colours are frequencies of light.
- White is the sum of all frequencies, and black is no frequency.
- The human retina has cones for red green and blue.
- The table:
  cyan    = white - red
  magneta = white - green
  yellow  = white - blue

With this we can explain all of the 'weird' things you mentioned:

> If you mix red and green light you get yellow.

If you mix red and green light your retina perceives all colours except 
blue, which is equal to yellow (see above).

> If you mix red and green paint you get brown.

True green paint absorbs red and blue and reflects green.  The "green" 
paint you are using probably reflects lots of red light and absorbs part 
of the green.

For instance, if you have paint that absorbs:
    - All of the blue light.
    - 50% of the red light.
    - 25% of the green light.
The paint will look perfectly green to you.  Try it in GIMP, this colour
corresponds to #80c000

Suppose that you mix this "green" with true red paint (absorbs blue and 
green and reflects red).  The resulting paint will absorb:
   - All of the blue light.
   - 25% of the red light.
   - 70% of the green light.

Thus, the colour reflects:
   - 75% of the red light.
   - 30% of the green light.

This colour corresponds to #c04c00.  Try it in GIMP, it's brown.

> If you mix all the colours of paint together you get something very dark.

True green paint absorbs blue and red.
True red paint absorbs green and blue.
True blue paoint absorbs green and red.

Mix these together and you get paint that absorbs 77% of the light from 
each of the three.  In other words, it reflects 33% of the light from all 

This colour is #545454.  Try it in GIMP, it's very dark (gray).  If you 
don't use pure red/green/blue paint you will not get exactly gray.  You 
will get some very dark colour.

> If you  paint a spinning top with segments of all colours and spin it you
> see something approaching white.

If the spinning top has equal segments of red green and blue, it will 
receive equal quantities of all three primary colours.  Thus it will look 
light (almost white).  The more intense the colours the closer to white.

If the colours are not pure, or are not equally represented, the final 
colour will shift from pure white as well.

> Hey. It's weird, isn't it?

I would call it "fascinating".

Note: I'm a scientist, I find this sort of thing fascinating.

Daniel Carrera
Graduate Teaching Assistant.  Math Dept.
University of Maryland.  (301) 405-5137
distrain: distrain (di-STRAYN) verb tr., intr.
   To seize the property in order to force payment for damages, debt, etc.

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