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

New Trends in Physics Teaching IV
In much the same way as a half wave antenna provides the best length to absorb and reradiate,
or scatter, radio waves, the best size for titanium dioxide particles is a half wavelength of light.
Figure 22 shows a plot of scattering versus wavelengths for different particle sizes. Very small
and very large particles do not scatter well. Suppose we mix a grey dye with our white scatterer,
as shown in figure 23. The grey absorbs equally throughout the spectrum. If the white pigment
scatters more in the red than the blue, the red light wil not penetrate very deeply into the paint
before it is scattered back out. On the other hand, blue light will penetrate more deeply before
it is backscattered, and in doing so wil be absorbed. Hence, the effect of mixing the white
pigment and the grey dye wil be to produce a red paint, a quite unexpected result.
Blue
Red
t
100per cent
Reflected
100per cent
White Pigment
with no absorption
Blue
Deep Penetration
1 leads to absorption
Red
very little
absorbed
White Pigment
I'
J
Figure 23. Scattering by a white pigment, undyed, and with a grey dye.
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