4.3.4 Polarisation

• 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
• 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
  • 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 called 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

Polarisation via Reflection

• When unpolarised light reflects from a smooth non-metallic surface, partial plane polarisation always occurs
• Reflected light is polarised in a plane parallel to the reflecting surface
  • This means if the surface is horizontal, a proportion of the reflected light will oscillate more in the horizontal plane than the vertical plane

• Polarising sunglasses use this property of reflection in order to reduce the glare coming from a reflective surface (e.g. water)

Polaroid sunglasses contain vertically oriented polarising filters which block out any horizontally polarised light

When sunlight reflects off a horizontal reflective surface (e.g. water) the light becomes horizontally polarised. This is where polaroid sunglasses come in useful with their vertically aligned filter

• As a result, objects under the surface of the water can be viewed more clearly

• Beams can be polarised, reflected or transmitted
• When beams are polarised, the oscillations of the waves are made to oscillate only in one plane
  • This affects the intensity of the waves
• Diagrams demonstrating polarisation will include a double-headed arrow showing the plane of polarisation of the wave

• When beams are reflected, they bounce back in the direction that they have come in by the same angle
• When beams are transmitted, they travel straight through the medium
  
  • In both these cases, the light can still be polarised
• Plane polarisation is when the direction of the vibrations stays constant over time, and the vibrations are 100 % restricted in that direction
• Partial polarisation is when there is some restriction to the direction of the vibrations but not 100 %
• This can be seen when an unpolarised light beam travels from air to glass
  • The light is initially unpolarised when incident on the glass
  • Some of the beam is reflected, partially polarising it
  • Some of the beam is transmitted and refracted, also partially polarising it