There are two main types of waves: longitudinal waves and transverse waves. Longitudinal waves involve compression and rarefaction of the transmitting medium, while transverse waves involve oscillation perpendicular to the direction of travel. Polarization is evidence of the transverse nature of waves - polarizers can absorb waves with certain orientations. Many applications make use of the polarized nature of transverse waves, such as sunglasses reducing glare by only transmitting vertically polarized light.

1. Longitudinal and transverse waves

2. Longitudinal Sound Seismic P-waves Compression waves on string Compressions Rarefactions

3. Transverse Water waves Waves on string Seismic S-waves EM waves No vibrations of particles in a transmitting medium Oscillating electric and magnetic fields c = 3 x 10 8 ms -1

5. Polarisation Evidence for transverse nature of waves Using a Polaroid to view light Using 2 Polaroids

6. Polaroid 1: only transmits waves in vertical plane Polaroid 2: only transmits waves in horizontal plane  absorbs all waves in other planes i.e. a polarised light detector Longitudinal waves – cannot be polarised

7. Light Unpolarised Plane polarised – oscillations restricted EM radiation = varying E and B field  polarisation of plane containing E field e.g. if light is vertically plane polarised, plane containing E field and direction of travel (plane of vibration) is vertical

8. Applications Sunglasses – reflected light partially plane polarised (sea/pond = horizontally plane polarised). Sunglasses only transmit vertically plane polarised light

9. Stress patterns – transparent polymers placed between crossed polarising filters Camera lenses – polarising film can reduce light intensity (and increase contrast) since light from sky is partially plane polarised (light from cloud is not)

10. Polarising light Polarising filters transmitted light plane polarised parallel to filter Polaroid sheet Light absorbed if polarised parallel to molecular alignment

11. Reflect it from transparent material Reflected ray – partially plane polarised parallel to surface (horizontal) Refracted ray partially plane polarised perpendicular to surface (vertical) If reflected ray perpendicular to refracted ray, both completely plane polarised (when angle incidence =  b Brewster’s angle) Sunglasses – can check if polarising by looking at reflected light

12. Scattering By suspension of small particles Scattered light partially plane polarised If scattered light at 90 ° to incident direction = completely plane polarised