Polarisation

Plane-polarised wave — one in which the oscillations occur in only one plane, containing the direction of energy travel.

Unpolarised wave — oscillations occur in all planes perpendicular to the direction of travel, changing rapidly and randomly. Light from a filament lamp or the Sun is unpolarised.

Only transverse waves can be polarised — this is the decisive test for distinguishing the two types. A longitudinal wave oscillates along its direction of travel, so there is no perpendicular plane to select: sound cannot be polarised, whatever you do to it.

Example: if an unknown wave can be polarised, it must be transverse. This is exactly how light was shown to be a transverse wave rather than a longitudinal one.

Polarising filters — transmit only the component of the oscillation parallel to the filter's transmission axis. Passing unpolarised light through one filter produces plane-polarised light at roughly half the original intensity, since only one of the two perpendicular components survives.

Crossed filters — a second filter at 90° to the first blocks everything, because no component of the already-polarised light lies along the second axis. Rotating the second filter varies the transmitted intensity from maximum at 0° to zero at 90°, and back to maximum at 180°.

Polarisation by reflection — light reflected from a non-metallic surface such as water or a road is partially polarised, mostly horizontally. Polaroid sunglasses have a vertical transmission axis, so they block that horizontally polarised glare while passing most other light. Tilting your head sideways while wearing them restores the glare, which is a quick way to demonstrate it.

Aerials and transmission — television and radio transmissions are polarised, so the receiving aerial must be aligned with the transmitter's polarisation. An aerial rotated through 90° receives almost nothing, which is why aerials in a region all point the same way.

Microwaves in the laboratory — a microwave transmitter and receiver with a metal grille between them demonstrate polarisation directly: rotating the grille changes the received signal from maximum to zero.