If
a beam of
converging rays, say, from a projector, is passed through a liquid
containing
minute particles in suspension, each of these particles scatters the
light rays
that fall on it, becoming, in a sense, a luminous point. Thus, the
entire path
of the rays through the liquid becomes visible. This gives the
appearance of a
bright cone when viewed in a darkened room. The Tyndall Effect can be
seen in
many examples in everyday life, such as headlight beams on foggy
nights, when sunlight comes through a window into a dusty room, or
comes down
through holes in clouds. An example of the Tyndall effect is shown
below in Fig 1.
Scattering
is a phenomenon in which the
direction, frequency, or polarization of a wave is changed when the
wave
encounters discontinuities in a medium, or interacts with material at
the
atomic or molecular level Fig 1.
It is a combination of reflection,
refraction and
diffraction. In the Tyndall effect the suspended
particles act
as the discontinuities in the medium which cause the light to be
scattered. When
the moment of the EM wave interacts with the particle, the electron
orbits
within the constituent molecules of the particle are perturbed
periodically
with the frequency of the electric field of the incident radiation. The
oscillation or perturbation acts as a source of EM radiation resulting
in a
scattering of the incident light.
Mie scattering theory takes into account both absorbing and non-absorbing spherical particles without any particular constraints on particle size. Mie theory has no size limitations, can be used for any medium and converges to the limits of geometrical optics for large particles. The theory accurately predicts the angle and intensity of the light scattered by the light-particle interaction.
The scattering
pattern produced will result in a
series of concentric bright and dark rings around a central maximum.
The image
of the scattering pattern is circularly symmetrical which means that
the
intensity of the scattered light will be equal in any direction for a
given
angle. This allows us to measure the scattering pattern in two
dimensions,
intensity and angle.
Fig 3. the
scattering pattern produced by a red laser. Note the pattern
is circularly symmetrical
