C11.7 Polarisation of EM Waves

EM waves: oscillating electric and magnetic field in direction of wave

Direction of Polarisation is the direction parallel to the electric field of the wave

Transverse waves can be plane polarised - only oscillating in one plane

Polarising filters can separate different waves. Can demonstrate this with polaroid filters (light)

Polaroid filters: plastic films containing very long this crystals. Direction of crystals limits transmission of polarised light in one direction only. Place another filter on another and rotate- transmitted light intensity falls until the 2 filters at 90 degrees when no light is transmitted

Unpolarised light: many oscillations in different directions. Pass lights through a polariser, then analyzer.

Check drawings

Microwaves produced artificially tend to be plane polarised. Any unpolarised microwaves polarised using metal grille. Inside door of microwave is a metal sheet with many holes - allows light to pass through but prevents microwaves escaping.

Check drawing

from surface:
- refracted light partially polarised perpendicular to surface
- reflected light partially polarised parallel to surface

When angle of incidence equals a specific angle, the reflected light is completely horizontally polarised

For air-water, its 53 degrees, for air-glass its 57 degrees.

This is why sunglasses remove glare of sunlight reflected from surface of roads

Polarisation examples: TV aerials pick up EM waves polarised in one plane . The way aerial is oriented makes a difference. Reduced interference from other sources.