7.5 Polarisation

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.

For plane-polarised light entering an analyser, transmitted intensity is I=I₀ cos²θ, where θ is the angle between the incident polarisation direction and analyser axis.

Use degrees or radians consistently, identify I₀ at θ=0 and remember that an analyser at 90° ideally blocks the light.

At θ=60°, the transmitted intensity is one quarter of I₀ because cos²60°=0.25.

The cosine is squared; using I=I₀cosθ overestimates transmission, and θ is not automatically the angle between two arbitrary filter surfaces.