Page 65 - Introduction to Colloid and Surface Chemistry
P. 65
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.
Light trap.
Powerful Neutral
light source filter
Sample Reference
©- -an
^•»a—-"^
Monochromafising
filter
P
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).
