Polarisation
- Transverse waves are waves with their displacement perpendicular to their direction of travel. These oscillations can happen in any plane perpendicular to the propagation direction
- Transverse waves can be polarised, this means:
- Vibrations are restricted to one direction
- These vibrations are still perpendicular to the direction of propagation/energy transfer
- The difference between unpolarised and polarised waves are shown in the diagram below
Diagram showing the displacement of unpolarised and polarised transverse waves
- Longitudinal waves (e.g. sound waves) cannot be polarised since they oscillate parallel to the direction of travel
- Waves can be polarised through a polariser or polarising filter. This only allows oscillations in a certain plane to be transmitted
Diagram showing an unpolarised and polarised wave travelling through polarisers
- Only unpolarised waves can be polarised as shown in diagram A
- When a polarised wave passes through a filter with a transmission axis perpendicular to the wave (diagram B), none of the wave will pass through
- Light can also be polarised through reflection, refraction and scattering
- An example of polarisation in everyday life is polaroid sunglasses. These reduce glare caused by sunlight for drivers to see through windows and fishermen to see beneath the water surface more clearly
Worked example
The following are statements about waves.Which statement below describes a situation in which polarisation should happen?
A. Radio waves pass through a metal grid
B. Surface water waves are diffractedC. Sound waves are reflected
D. Ultrasound waves pass through a metal gridANSWER: A
- Polarisation only occurs for transverse waves, therefore, C and D can be ruled out as sound and ultrasound are both longitudinal waves
- Waves are not polarised when diffracted, hence we can also rule out option B
- Radio waves are transverse waves - they can be polarised by a metal grid so only the waves that fit through the grid will be transmitted, therefore, A is correct