In this topic of IBDP Physics, we will learn about polarisation.
Polarisation
- Transverse waves only.
- Unpolarised waves – plane of wave oscillation is randomly orientated.
- Plane polarised waves – all waves have matching planes of oscillation e.g. vertical or horizontal.
In this example, the polariser’s “preferred plane of polarisation” is vertical.
This means it will only let through vertically polarised waves and absorb the rest.
- In IBDP Physics, a polariser for microwaves is made from parallel metal bars.
- A polariser for optical wavelengths is made from parallel polymer chains.
- Charged particles are free to oscillate along the metal bars (or polymer chains)
- So they absorb electromagnetic waves which oscillate parallel to those bars/chains
Aerials
Examples of polarised (selective) absorbers – Short horizontal metal bars on television aerials. (Therefore TV broadcasts are horizontally plane polarised)
Sunglasses
- In IBDP Physics, polymer chains in certain plastics – Polaroid sunglasses.
- Reflected glare from horizontal surfaces is horizontally plane polarised.
- Horizontal polymer chains in polarising sunglasses only let through vertically polarised light (and so block the glare)
Malus’s Law
In IBDP Physics, if the angle of rotation between two parallel polarisers is θ then the Intensity, I of the transmitted light is given by:
I =Io Cos2θ
Where Io is the incident intensity on the second polariser.
(Malus’s Law is no longer on the spec but you will still find it in past exam questions)
This is the end of this topic.
