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

y move in the direc
value of the wavelength on the periphery of the chromaticity diagram.
Scattering
mixing dyes, with no scattering,
With mixtures of paints, however, s
simpIe-subtractive, can therefore be
e very important.
Scattering introduces yet ther complication. One of the principal criteria of a paint which
invoIves scattering is good ering’ or ‘hiding’ PO y the mass of paint per
unit area to obscure a black and white chequerb
Except for black, or very dark
paints, the covering power is determined in part b
g power of the pigment particles.
In the past, this was often lead white (basic lead carbonate, refractive index 1.94-2.09) in linseed
oil (refractive index 1.48). The poisonous nature of the lead salt caused it to be replaced by
titanium dioxide. The oxide is of high refractive index (2.6) but quite transparent. Hence, it
acts as a white paint purely by virtue of scattering all the wavelengths of light, for which its
refractive index must be very different from the medium in which it is embedded. It is for a
similar reason that ground glass looks whit;.
0 0.5 1.0 1.5
Particle size/wavelength
Figure 22. Scattering as a function of particle size for titanium dioxide.
215