Why does a mixture of red and green paints produce brown, while a mixture of the particular wavelengths of light corresponding to red and to green produces yellow? In other words, what is the difference between these two processes which is responsible for their surprisingly different outcomes? The difference between additive and subtractive color mixing may be very easily explained in terms of what we have learned thus far about the visible part of the electromagnetic wavelength spectrum and about the anatomy and functioning of the eye. Mixing paint is an example of subtractive color mixing, in which certain wavelengths of light are blocked out from the wavelength mixture before ever reaching the eye. Taking as an example a green and a red pigment: a green pigment might reflect all wavelengths of light from about 550 to 650 nm, and absorb all wavelengths of light from about 400-550 nm and 650-700 nm. A red pigment might reflect all wavelengths from about 625 to 700 nm, and absorb all wavelengths below 625 nm. When these two pigments are mixed, their absorbtions work against one another so that only a restricted range of wavelengths are reflected:

400 450 500 550 600 650 700

| | 400 450 500 550| | 650 700 GREEN 400 450 500 550| |625 650 700 RED

| |

\ / \ /

551 - 624

The wavelengths that are reflected correspond to neither of the two original colors in isolation, and are perceived as having a "new" color (in this case, brown). Mixing light is an example of additive color mixing, and is different from subtrative color mixing in that all mixing is being done in the eye itself. Continuing with the example of green and red: a "green" light might contain all wavelengths of light from about 550 to 650 nm, while a "red" light might contain all wavelengths from about 625 to 700 nm. When these two lights are shone onto one spot on a wall, all the wavelgths that compose both lights individually are refected into the eye simultaneously. That is,

  
                        550     600     650             GREEN
 
                       |       |   625 650     700     RED
 
                       |       |   |   |         |
 
                       |       |   |   |         |
 
                      \ /     \ / \ / \ /       \ /
 
                       550 ------------------- 700

The reason why yellow in particular is seen is rather complicated, having to do with the layout of the spectrum. However, the logic behind the difference between additive and subtractive color mixing in color vision is very clear and simple.

VERY nice explanation of the difference between additive and subtractive color mixing, which I didn't have time to go into in class (and probably wouldn't, in any case, have done so clearly). Have thought about doing something which involves teaching as a career? Can be VERY satisfying to the right sort of brain (one like mine), and seems to me a very important thing to be doing in terms of current social/cultural needs. PG