The experiments described earlier have shown effects which are characteristic of waves. These effects will occur both with transverse and longitudinal waves. The phenomenon of POLARIZATION, however, only occurs with transverse waves.
Consider the transverse waves which are set up in a rope. These waves may have any orientation.
If a cardboard slit is placed over the rope, as shown, then the waves will be constrained to the plane of the slit:
The waves which emerge from the slit are plane-polarized and the slit is called the polarizer.
If a second slit (called the analyzer) is placed over the rope parallel to the polariser, then the wave will pass through it undisturbed:
If the analyser is now rotated 90º so that it is perpendicular to the polariser, then the waves can no longer pass along the rope. In this case the slits are said to be crossed.
Crossed slits will only prevent the propagation of a transverse wave and have no effect on the propagation of a longitudinal wave.
Transparent sheets can be made by depositing crystals of quinine iodosulphate uniformly on a cellulose film in such a way that the crystal axes are aligned with one another. This material is called polaroid. It has the property that light is transmitted through two sheets when their axes are aligned, but not when the axes are crossed.
This phenomenon can be explained if light has TRANSVERSE WAVE properties. The first sheet acts as a polarizer limiting the oscillations to one plane (i.e., it polarizes the light).The second sheet acts as an analyzer, passing the light when it is aligned with the axis of the polarizer, but not when it is crossed. Since light can be polarized, light waves must be transverse and not longitudinal.