Polarisation

The definition of transverse waves tells us that oscillations take place in a direction perpendicular to the direction of propagation. But there are infinitely many such directions.

Normally, oscillations in many different planes (all perpendicular to the direction of propagation, of course) may coexist in a wave. We call such waves unpolarised. In the video, the motor produces such waves.

The first slit only transmits waves whose oscillations are parallel to it. This is called a polariser. Having passed through this slit, the all oscillations in the wave are in a single plane — this type of wave is called a linearly polarised wave. The second slit — in arrangements with two polarisers in variable orientation relative to each other, the second one is called an analyser — only transmits oscillations in the horizontal plane. Since the polarised wave no longer contains such, there will be no wave motion whatsoever after the analyser.

In addition to linear polarisation, there are more complex forms of polarisation, such as circular or elliptical polarisation.