CHAP. 14: DISPERSION SYSTEMS [CONTENTS] 460
- The scattering is inversely proportional to the fourth power of the wavelength of the
incident light.
Example
Explain why the sky is blue and the setting sun is red.Solution
The sunlight is scattered in the atmosphere by way of the Rayleigh scattering. Since the blue
light has a shorter wavelength, it is scattered more and the sky is blue. If we observe the sun
directly, we observe unscattered light which has a complementary red colour. This effect is evident
particularly during the sunrise and sunset when the sunlight has to pass through a thick layer of
the atmosphere.14.2.2 Sedimentation of colloid particles
Another effect observed in colloid systems is their gradual sedimentation and attainment of a
sedimentation equilibrium.
The rate ofsedimentation in the gravitational filed,v, for sedimenting spherical par-
ticles of the radiusris given by the following relation (provided that Stoke’s lawFresistance=
6 π r η v)
v=dh
dτ=
m g
6 π η r(
1 −ρ 0
ρ)
=2
9
g r^2
η(ρ−ρ 0 ), (14.4)wherem= (4/3)π r^3 is the mass of the spherical particle,ρis the particle density,ρ 0 is the
density of the dispersion environment,ηis its viscosity andgis the gravitational acceleration.
Example
Calculate the sedimentation rate of quartz particles of the radius 5 × 10 −^6 m in water at 25◦C
(η= 1. mPa s,ρ 0 = 1. g cm−^3 ). The quartz density is 2. 6 g cm−^3.