Page 67 - Introduction to Colloid and Surface Chemistry

Optical properties 57

The quantity R e (1+cos 2 0) is called the Rayleigh ratio. The unity
2
term in (1+cos 0) refers to the vertically polarised component of the
scattered light, and the cos 2 0 term to the horizontally polarised
component.
4
Since the scattering intensity is proportional to I/A , blue light (A ~
450 nm) is scattered much more than red light (A ~ 650 nin). With
incident white light, a scattering material will, therefore, tend to
appear to be blue when viewed at right angles to the incident beam
and red when viewed from end-on. This phenomenon is evident in
the blue colour of the sky, tobacco smoke, diluted milk, etc., and in
the yellowish-red of the rising and setting sun.

interparticle interference

If the scattering sources in a system are close together and regularly
spaced, as in a crystalline material, there will be regular phase
relationships (coherent scattering) and, therefore, almost total
destructive interference between the scattered light waves - i.e. the
intensity of the resulting scattered light will be almost zero. When the
scattering sources are randomly arranged, which is virtually the case
for gases, pure liquids and dilute solutions or dispersions, there are
no definite phase relationships (incoherent scattering) and destructive
interference between the scattered light waves is incomplete.
For a system of independent scatterers (point sources of scattered
light distributed completely at random), the emitted light waves have
an equal probability of reinforcing or destructively interfering with
one another. The amplitudes of the scattered waves add and subtract
in a random fashion, with the result that (by analogy with Brownian
displacement; page 25) the amplitude of the total scattered light is
proportional to the square root of the number of scattering particles.
Since the intensity of a light wave is proportional to the square of its
amplitude, the total intensity of scattered light is proportional to the
number of particles.

Relative molecular masses from light-scattering measurements

If the dimensions of a scattering particle are all less than c. A/20, the
scattered light waves emanating from the various parts of the particle
cannot be more than c. A/10 out of phase, and so their amplitudes are
practically additive. The total amplitude of the light scattered from