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for two dipoles. The scattered intensity in the forward direction increases rapidly and becomes
more sharply peaked as the number of dipoles increases. This is illustrated in Figure 2, which
shows the intensity of the scattered light as a function of scattering angle for one, two and
four dipoles in a line. In the case of two and four dipoles, the incident wave for each dipole
is composed of the external incident wave and the scattered waves from the other dipoles. The
dipoles are coupled; they "see" the electric fields of the neighbouring dipoles.
As particle shape changes, the relative locations of the dipoles comprising the particle change
and the phase relationships among the scattered wavelets change. In the forward direction,
however, the waves are still in phase and the scattered intensity in that direction is least
sensitive to particle shape. The same idea applies to particle orientation. To examine particle
shape with light scattering, we need to place our detector at scattering angles away from the
forward direction. The size of the scattering angle for best sensitivity to particle shape is
determined empirically and depends on the size and shape of the particle.
The optical composition of a particle is specified by its refractive index relative to the
surrounding medium. The polarizability, which is the magnitude of the induced dipole moment
per unit exciting field, is the microscopic counterpart of the macroscopic refractive index. The
amplitude of each scattered wavelet is larger for larger dipole polarizabilities. As a result the
interaction or coupling among the dipoles is strong for particles composed of dipoles with
large polarizabilities. At visible wavelengths, biological particles have low relative refractive
indices and are considered to be composed of weakly interacting dipoles.
The wavelength and polarization state are the important characteristics of the incident light.
The wavelength determines the scale for the particle size. The polarization state specifies the
direction of the electric field of the incident wave as shown in Figure 3. The field is
perpendicular to the direction of the wave. This light is linearly polarized because the electric
field vectors all lie in a fixed plane. The amount of light scattered by a dipole depends on the
orientation of the electric field of the incident wave with respect to the scattering plane. The
scattering plane is defined by the direction of the incident wave and a line extending from the
particle toward a detector. For light polarized perpendicular to the scattering plane, the
induced dipole is also perpendicular to that plane and the intensity of the scattered light is the
for two dipoles. The scattered intensity in the forward direction increases rapidly and becomes
more sharply peaked as the number of dipoles increases. This is illustrated in Figure 2, which
shows the intensity of the scattered light as a function of scattering angle for one, two and
four dipoles in a line. In the case of two and four dipoles, the incident wave for each dipole
is composed of the external incident wave and the scattered waves from the other dipoles. The
dipoles are coupled; they "see" the electric fields of the neighbouring dipoles.
As particle shape changes, the relative locations of the dipoles comprising the particle change
and the phase relationships among the scattered wavelets change. In the forward direction,
however, the waves are still in phase and the scattered intensity in that direction is least
sensitive to particle shape. The same idea applies to particle orientation. To examine particle
shape with light scattering, we need to place our detector at scattering angles away from the
forward direction. The size of the scattering angle for best sensitivity to particle shape is
determined empirically and depends on the size and shape of the particle.
The optical composition of a particle is specified by its refractive index relative to the
surrounding medium. The polarizability, which is the magnitude of the induced dipole moment
per unit exciting field, is the microscopic counterpart of the macroscopic refractive index. The
amplitude of each scattered wavelet is larger for larger dipole polarizabilities. As a result the
interaction or coupling among the dipoles is strong for particles composed of dipoles with
large polarizabilities. At visible wavelengths, biological particles have low relative refractive
indices and are considered to be composed of weakly interacting dipoles.
The wavelength and polarization state are the important characteristics of the incident light.
The wavelength determines the scale for the particle size. The polarization state specifies the
direction of the electric field of the incident wave as shown in Figure 3. The field is
perpendicular to the direction of the wave. This light is linearly polarized because the electric
field vectors all lie in a fixed plane. The amount of light scattered by a dipole depends on the
orientation of the electric field of the incident wave with respect to the scattering plane. The
scattering plane is defined by the direction of the incident wave and a line extending from the
particle toward a detector. For light polarized perpendicular to the scattering plane, the
induced dipole is also perpendicular to that plane and the intensity of the scattered light is the