Page 66 - Introduction to Colloid and Surface Chemistry

56 Optical properties
2, Debye scattering (where the particles are relatively large, but the
difference between their refractive index and that of the dispersion
medium is small).
3. Mie scattering (where the particles are relatively large and have a
refractive index significantly different from that of the dispersion
medium).

Scattering by smafl particles

Rayleigh (1871) laid the foundation of light-scattering theory by
applying the electromagnetic theory of light to the scattering by
small, non-absorbing (insulating), spherical particles in a gaseous
medium. When an electromagnetic wave of intensity / 0 and wave-
length A falls on a small (< c. A/20) particle of polarisability a,
oscillating dipoles are induced in the particle. The particle then serves
as a secondary source for the emission of scattered radiation of the
same wavelength as the incident light. For an unpolarised incident
beam, the intensity / e at a distance r from the particle of the light
scattered at an angle 6 to the incident beam is given by the expression

(3.3)

Unpolarised
incident
beam

Figure 3.6 Radiation envelope for light scattered from small particles. Distances
from the origin of the dotted, dashed and smooth lines represent the relative intensities
of the horizontally polarised component, vertically polarised component and total
scattered light, respectively