11.7 Polarisation of electromagnetic waves Flashcards
Most naturally occuring…
Unpolarised…
…electromagnetic waves are unpolarised. The electric field oscillates in random planes, all at 90 degrees to the direction of energy transfer.
…electromagnetic waves can be polarised using filters called polarisers. The nature of the polariser depends on the part of the electromagnetic spectrum to be polarised, but each filter only allows waves with a particular orientation through.
What are polaroid filters?
Plastic films that contain very long thin crystals and polarise light.
They are used in sunglasses, and over liquid crystal displays such as watches.
Unpolarised light passing…
..through the first filter becomes plane polarised.
If the second filter (sometimes called the analyser) is in the same plane as the first, then the light passes through unaffected.
However, if the second polaroid is slowly rotated, the intensity of the light transmitted drops. When the second filter has turned through 90 degrees, no light is transmitted and the intensity falls to zero.
What is a use of polarisation in electromagnetic waves?
In communication transmitters.
In order to reduce interference between different transmitters, some transmit vertically plane polarised waves and others nearby transmit horizontally plane polarised waves.
An aerial aligned to detect vertically polarised radio waves will suffer less interference from horizontally polarised waves and vice versa.
