Scattering

Unpolarized light is incident on a molecule. The $\vec{E}$ field of the incident light induces a dipole moment in the molecule. This oscillating dipole sends out radiation in different directions. This is the process of scattering.

Figure 16.6: Scattering of light by a molecule

The incident light can be decomposed into two orthogonal linear polarizations respectively parallel and perpendicular to the plane of the page as shown in Figure 16.6. An observer O located at $90^{\circ}$ to the incident direction observes the scattered light. The dipole produced by the parallel component of the incident $\vec{E}$ does not produce any $\vec{E}$ at the observer O. The observer O receives radiation only from the dipole oscillations perpendicular to the plane of the paper. The scattered radiation received at O is linearly polarized perpendicular to the plane of the paper.

During daytime the sky appears illuminated because of the sunlight scattered by the atmosphere (Figure 16.7).

Figure 16.7: Polarization by scattering

The scattered light is polarized by the mechanism discussed here. The polarization is maximum (although not completely plane polarized) when the light is scattered at $90^{\circ}$ to the incident direction. It is less partially polarized at other scattering angles.