7.5.1—Polarisation is a phenomenon associated with transverse waves

Polarisation restricts oscillations to a preferred direction. Only transverse waves can be polarised because longitudinal oscillations have no sideways direction to select.

Describe the transmission axis and the orientation of the incident electric field. A polariser changes allowed vibration direction, not necessarily frequency.

A polarising filter can reduce light intensity as it is rotated relative to the incoming plane-polarised light.

Polarisation is not the same as reflection, and observing polarisation rules out a purely longitudinal wave.