which can thus pass the emulsion almost unscattered. For still larger
particles, the dependence of scattering onlis very small, and the emulsion
(or other type of dispersion) appears white, the more so if the particle
concentration is larger.
Absorption. The possible color of the last mentioned emulsion
needs further explanation. Up till now, we have implicitly assumed that light
is only scattered, not absorbed. If absorption occurs, we should use the
complex refractive indexnn~¼nin^0 , wheren^0 determines the adsorption
ði¼H 1 Þ. The relations now become more complicated.n^0 is related to the
specific extinctiongaccording tog¼ 4 pn^0 =l. The absorbency as determined
in a spectrophotometer equals 0: 434 gL, whereLis the optical path length.
Generally,n^0 is far more strongly dependent on wavelength than isn, and it
gives rise to a fairly narrow absorption peak as a function of l;
consequently, we see the material to be colored if the absorption occurs
for visible light. Assume that we have a strong absorbency, being unity for
an optical path length of one mm atl¼ 0 : 55 mm. We now calculate thatn^0 ¼
0.0001. This meansn^05 jn 1 j, and under such conditions scattering tends
to predominate over absorption; hence the emulsion still looks white.
However, if the drops are large, the scattering per unit mass of oil becomes
small (Figure 9.8), and now absorption can be perceived. The yellow color
of an oil will then give a somewhat creamy color to the emulsion. Drops
of mm size can, of course, be seen as such, the color included.
Diffuse Reflection. Light scattering by homogeneous small
particles of fairly simple shape can accurately be calculated. Measurement
of scattering by a dispersion can therefore yield information on
concentration, size, and possibly shape of the particles. However, this is
straightforward only if the dispersion is dilute. For concentrated systems,
multiple scatteringoccurs, which means that light scattered by one particle is
subsequently scattered by many others. Moreover,interferenceoccurs of the
light scattered by particles that are close to each other, and some structural
elements may absorb light. In most disperse foods, all these phenomena will
happen. If light falls onto such a system, some light will be directly reflected,
but a much greater part will penetrate and be scattered numerous times at
the surfaces of structural elements, before it emerges again; this is called
diffuse reflection. In the meantime, some of the light will often be absorbed,
generally at specific wavelengths, giving rise to color. The phenomena
involved generally are far too complicated to calculate the amount and
wavelength dependency of the diffusely reflected light. Consequently, diffuse
reflection by food systems has to be measured and can hardly be predicted,
if at all. If the food contains substances that absorb light at certain