New trends in physics teaching, v.4; The ... - unesdoc - Unesco

New Trends in Physics Teaching IV
Artists versus physicists - the primary colours
Artists are vehement that red, yellow and blue are the three primary colours, whereas physicists
believe these are red, green and blue. If we assume that a bluish-turquoise is called ‘blue’ by
artists, and a reddish-magenta ‘red’, we could say that in fact, artists were merely employing the
negative primaries, and paints behaved like dyes. Unfortunately, paints are not that simple.
Figure 24 shows what happens when we mix differing quantities of blue, red and green, and blue,
red and yellow poster paints in pairs. As will be seen, the area of the CIE diagram circumscribed
by these mixtures is much larger with yellow than with green. Although paints behave largely
as dyes, and hence act as subtractive filters, nevertheless, there is also an additive component.
This may be seen if we imagine the pigment particles to be completely opaque. Then, looking
at the surface, we would see a series of dots of one colour or another, which would combine
additively in the retina. Seurat made use of this effect in his ‘Pointillist’ paintings composed
of little dots of paint, which appear separate when examined close, but combine when we move
back to view the picture.
Cornea
S
Figure 25. Horizontal cross section of the right eye.
THE PHYSIOLOGY OF COLOUR VISION
The physics, physiology and psychology of the eye are interwoven. The physical part of vision
consists largely of focusing the object being viewed onto the retina, in much the same way as
does a camera. Most of the refraction or bending of light as it enters the eye occurs at the cornea,
as shown in figure 25. This is because the refractive index of the main body of the eye, being
composed of an aqueous fluid, is approximately 1.33. The cornea itself has a refractive index of
1.37 and a radius of curvature approximately 7.8 mm. The light passes through the aqueous
ugh the so-called crystalline
ed by the fluids contained
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