Polarisation
Polarisation of transverse waves
- Transverse waves are waves that oscillate with their displacement perpendicular to their direction of travel
- These oscillations can happen in any plane perpendicular to the propagation direction
- An electromagnetic wave is an example of a transverse wave that oscillates in more than one plane
- A magnetic field oscillates on one plane
- An electric field oscillates on a different plane
- Both planes are at right angles to each other and to the direction of wave motion
An electromagnetic wave is generated by the combined oscillation of an electric and a magnetic field
- When transverse waves are polarised
- vibrations are restricted to one plane of oscillation
- vibrations are still perpendicular to the direction of propagation / energy transfer
- The difference between unpolarised and polarised waves are shown in the diagram below
- The blue arrows represent the direction of the plane of oscillation of the transverse wave
- An unpolarised wave oscillates in many planes of oscillation
- A vertically polarised wave oscillates only in the vertical plane
- A horizontally polarised wave oscillates only in the horizontal plane
Different planes of polarisation
Diagram showing the displacement of unpolarised and polarised transverse waves
Polarising filters
- Waves can be polarised by passing through a polariser or polarising filter.
- Polarisers only allow oscillations in a certain plane to be transmitted through the filter
- A polariser is an optical layer or lens that consists of many parallel tiny slits
- A polariser only allows waves vibrating parallel to the direction of these slits to pass through
- Waves that are not orientated in the correct plane are blocked by the filter and do not pass through
Action of a polarising filter
Diagram showing an unpolarised and polarised wave travelling through polarisers
- Only unpolarised waves can be polarised
- This is 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
Polaroid sunglasses
- Polaroid sunglasses are an example of a polarisation
- Polaroid sunglasses contain lenses with polarising filters with transmission axes that are vertically oriented
- This means the glasses do not allow any horizontally polarised light to pass through
Polaroid sunglasses
Polaroid sunglasses contain vertically oriented polarising filters which block out any horizontally polarised light
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 diffracted
C. Sound waves are reflected
D. Ultrasound waves pass through a metal grid
Answer: 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