W9_parallel_resonance.eps

Week 11: Light 385

incident ray

scattered rays

Figure 151: The scattering of initially unpolarized light by a molecule or dust particle. Note that the
polarization is perpendicular to theplane of scatteringfor each of the possible outgoing directions.

Polarization by Scattering

When unpolarized light passes across an atom or molecule, itpolarizesit in the instan-

taneous direction of the electric field vector (which, recall, has a definite direction at any

time but which jumps around to a new direction every 10-1000 optical periods). The

oscillating molecule acts like adipole antennaandreradiatesthe incident electromagnetic

wave. However, the reradiated electric field must beparallelto the dipole moment of

the molecule, and there is no radiationalongthe dipole (with a clear maximum at right

angles to the dipole. As a consequence we can easily see that the rulefor polarization of

rays scattered more or less at right angles is that they must be polarizedperpendicular

to the plane of scattering!

Polarization by Reflection

φ

θ

n 1

n 2

Figure 152: The scattering of initially unpolarized light by reflection offof a plane surface between
two dielectric media at theBrewster anglethat produces complete polarization of the reflected ray.
Note that the polarization of all reflected rays incident on the surface at an angle isparallel to the
groundeven at angles other than the Brewster angle.

When light strikes a surface between two regions with differing indicesof refraction, it

is partially transmitted and partially reflected (with the amount of each determined by

the angle of incidence and the two indices of refraction). The reflection is caused by the

polarization of surface molecules in such a way that the light scattered by them adds up