Polarization of Light

1. Introduction

An ordinary (unpolarized) light beam consists of electric-field vibrations in all directions perpendicular to the direction of propagation. Polarization is the process of restricting these vibrations to a single plane. Light in which the electric field vibrates in only one plane is called plane-polarized light.

Because polarization can only occur for transverse waves (whose vibrations are perpendicular to propagation), the fact that light can be polarized is direct proof that light is a transverse wave.

2. Polarization and the Transverse Nature of Light

• Transverse waves (vibrations perpendicular to propagation, like light) can be polarized: a polarizer selects one plane of vibration and blocks the rest.
• Longitudinal waves (vibrations along the direction of propagation, like sound) cannot be polarized, because there is no perpendicular plane to select.

Experimental argument: if two polarizers (a polarizer and an analyser) are placed one behind the other and the analyser is rotated, the transmitted intensity varies and falls to zero when the axes are crossed at $90^\circ$. If light were longitudinal, rotating the analyser would have no effect. The observed extinction can only happen if the vibrations are perpendicular to the direction of propagation, which proves light is transverse.

3. Methods of Producing Polarized Light

1. By reflection (Brewster's method): when unpolarized light is reflected from a transparent surface at a particular angle (Brewster's angle $i_p$), the reflected light is completely plane-polarized.
2. By refraction (pile of plates): light refracted through a stack of glass plates at Brewster's angle becomes increasingly polarized as it passes through each plate.
3. By scattering: light scattered by small particles (for example the blue sky) is partially polarized; light scattered at $90^\circ$ to the incident beam is completely polarized.
4. By selective absorption (Polaroid): the most practical method, using a sheet that transmits vibrations in only one direction (see Section 6).