Fr.: diffusion Rayleigh
The scattering of light by → particles
of size small compared with the → wavelength of
light. The intensity of the light scattered by unit volume of the medium at an
angle θ to the direction of propagation of the incident light is:
Iθ = 8 π4α2 N I0
(1 + cos2θ)/(R2λ4),
where α is the → molecular polarizability,
N is the number of scattering molecules,
I0 is intensity of the incident light, λ is the wavelength, and
R is the distance from the scatterer.
The fourth power dependence on wavelength means that blue light is
much more strongly scattered than red light from a medium containing very fine particles.
The air molecules, mostly → nitrogen (78%) and
→ oxygen (21%) are some 1,000 times larger than
→ visible light wavelengths.
This accounts for the bluish appearance of smoke and of clear sky when the observation is not
along the direction of illumination. The setting Sun, seen through a considerable
thickness of atmosphere appears reddish because long wave radiation predominates in
the transmitted light.