Polarisation (AQA A Level Physics): Revision Note

Polarisation

• Polarisation is when:

Particle oscillations occur in a single plane perpendicular to the direction of wave propagation

• Polarisation can only occur in transverse waves

  • This is because transverse waves oscillate in any plane perpendicular to the propagation direction

• When transverse waves are 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 is shown in the diagram below:

• Longitudinal waves (e.g. sound waves) cannot be polarised

  • This is because 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 A shows:

  • Only unpolarised waves can be polarised

• Diagram B shows:

  • When a polarised wave passes through a filter with a transmission axis perpendicular to the wave, none of the wave will pass through

• Light can also be polarised through reflection, refraction and scattering

Investigating light intensity with two polarisers

• If an unpolarised light source is placed in front of two identical polarising filters, A and B, with their transmission axes parallel:

  • Filter A will polarise the light in a certain axis

  • All of the polarised light will pass through filter B unaffected

  • In this case, the maximum intensity of light is transmitted

• As the polarising filter B is rotated anticlockwise, the intensity of the light observed changes periodically depending on the angle B is rotated through

• When A and B have their transmission axes perpendicular to each other:

  • Filter A will polarise the light in a certain axis

  • This time none of the polarised light will pass through filter B

  • In this case, the minimum intensity of light is transmitted

• The resulting graph of the light intensity with angle, as the second polariser is rotated through 360°, looks as follows:

• In real life, the intensity of the unpolarised electromagnetic wave reduces after it passes through a polarising filter

Applications of polarisers

Polaroid sunglasses

• Polaroid sunglasses are glasses containing lens with polarising filters with transmission axes that are vertically oriented

  • This means the glasses do not allow any horizontally polarised light to pass through

• When light is reflected from a reflective surface e.g. the surface of water or a wet road, it undergoes partial plane polarisation

  • This means if the surface is horizontal, a proportion of the reflected light will oscillate more in the horizontal plane than in the vertical plane

• Therefore, polaroid sunglasses are useful in reducing the glare on the surface of the water (or any reflective surface) as the partially-polarised light will be eliminated by the polarising filter

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

Polaroid photography

• Polaroid cameras work in the same way as Polaroid sunglasses

  • They are very useful for capturing intensified colour and reducing glare on particularly bright sunny days

• Polarising filters also enable photographers to take photos of objects underwater

  • This is because the light reflected on the surface of the water is partially polarised in the horizontal plane

  • This glare is eliminated by the polarising lens

  • However, the light from the underwater object is refracted by the surface of the water, not reflected, so it is not plane-polarised

  • Therefore, the light from the underwater object is more intense than the glare and shows up much more brightly in the photo

Polarisation of radio and microwave signals

• Radio and television services are broadcast either horizontally-polarised or vertically-polarised

• Therefore, the reception aerial needs to be mounted flat (horizontal), or on its side (vertical):

  • The particular orientation of an aerial will depend on the transmitter it is pointing towards and the polarity of the services being broadcast