Plane Polarisation (Edexcel AS Physics)

• Transverse waves can oscillate in any plane perpendicular to the direction of motion (and energy transfer) of the wave
• Such waves are said to be unpolarised
• Polarisation occurs when

Particles are only allowed to oscillate in one of the directions perpendicular to the direction of wave propagation

• When a transverse wave is polarised, its electric field is only allowed to oscillate in one fixed plane perpendicular to the direction of motion of the wave
  • For EM waves it is the plane of the electric fields oscillation that defines its plane of polarisation
•  A transverse wave can be vertically polarised, horizontally polarised, or polarised in any direction in between

Diagram showing the displacement of unpolarised and polarised transverse waves

• Since longitudinal waves oscillate in the same direction as the direction of motion of the wave, polarisation of longitudinal waves cannot occur 

• Methods of polarisation include polarising filters and reflection from a non-metallic plane surface

Polarising Filters

• Light waves can be polarised by making them pass through a polarising filter (also known as a polariser)
• The filter imposes its plane of polarisation on the incident light wave
• A polariser with a vertical transmission axis only allows vertical oscillations to be transmitted through the filter (A)
• If vertically polarised light is incident on a filter with a horizontal transmission axis, no transmission occurs (B), and the wave is blocked completely

Diagram showing an unpolarised and polarised wave travelling through polarisers