Longitudinal and transverse waves

Longitudinal

• Sound• Seismic P-waves• Compression waves on string

• Compressions• Rarefactions

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 108 ms-1

Polarisation

• Evidence for transverse nature of waves

• Using a Polaroid to view light• Using 2 Polaroids

• 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

Light

• Unpolarised• Plane polarised – oscillations

restricted• EM radiation = varying E and B field

polarisation of plane containing E fielde.g. if light is vertically plane polarised, plane containing E field and direction of travel (plane of vibration) is vertical

Applications

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

• 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)

Polarising light1. Polarising filters

• transmitted light plane polarised parallel to filter

2. Polaroid sheet• Light absorbed if polarised parallel to molecular

alignment

3. 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

4. Scattering• By suspension of small particles• Scattered light partially plane

polarised• If scattered light at 90° to incident

direction = completely plane polarised