92168.pdf

Optical properties 5 5

The intensity of the light scattered by colloidal solutions or
dispersions of low turbidity is measured directly. A detecting
photocell is mounted on a rotating arm to permit measurement of the
light scattered at several angles, and fitted with a polaroid for
observing the polarisation of the scattered light (see Figure 3.5).
Weakening of the scattered beam itself as it passes through the
slightly turbid sample can be neglected, and its intensity can be
compared with that of the transmitted beam.

Powerful Neutral

light source filter

Monochromafising

filter

P

Light trap.

Sample Reference

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Nicol prism /

Photomultiplier tube

Figure 3.5 Measurement of scattered light

Although simple in principle, light-scattering measurements present a

number of experimental difficulties, the most notable being the

necessity to free the sample from impurities such as dust, the

relatively large particles of which would scatter light strongly and

introduce serious errors.

Light-scattering theory

It is convenient to divide the scattering of light by independent

particles into three classes:

1. Rayleigh scattering (where the scattering particles are small
   enough to act as point sources of scattered light).