4.7.2Electrons, Waves and Photons
Polarisation
5 min read 4 parts Lesson 48 of 62 Unit quiz Virtual lab
Section 1 of 4Plane Polarised Waves
Polarisation shows that light and other electromagnetic waves oscillate across their direction of travel.
Unpolarised wave
- An unpolarised transverse wave has oscillations in many planes, all perpendicular to the direction of travel.
Plane polarised
- In a plane polarised wave the oscillations are restricted to one single plane.
Transverse only
- Only transverse waves can be polarised,
- because longitudinal oscillations lie along the direction of travel and have no plane to restrict.
Evidence
- Polarisation is therefore evidence that light is a transverse wave.
Section 2 of 4Polarising Filters
1
A polarising filter lets through only the component of oscillation along its transmission axis.
2
Unpolarised light passing through one filter emerges plane polarised and with reduced intensity, about half of the original.
3
A second filter, the analyser, is placed after the first and rotated.
4
When the two transmission axes are parallel, the transmitted intensity is greatest; when they are at 90°, almost no light passes.
5
Rotating the analyser through a full turn produces two bright positions and two dark ones.
Section 3 of 4Applications
Applications
Sunglasses
- Polarising lenses block reflected glare from roads and water, because reflected light is partly polarised.
Aerials
- Television and radio aerials are rotated to line up with the polarisation of the transmitted waves for best reception.
Photography
- Polarising filters on cameras reduce reflections and darken skies for clearer pictures.
Stress analysis
- Transparent plastic models between polarisers show patterns that reveal where stress is concentrated.
Section 4 of 4Exam Checklist & Tips
4.7.2 – Key Points for the Exam
- Polarisation Restricting a transverse wave's oscillations to a single plane.
- Plane polarised wave A wave whose oscillations occur in one plane only.
- Transmission axis The direction along which a polarising filter allows oscillations through.
- Analyser The second polarising filter used to test whether light is polarised.
Common Mistakes
- Claiming sound waves can be polarised; they are longitudinal.
- Saying a polarising filter has holes that physically filter the waves like a sieve.
- Forgetting that the intensity drops when unpolarised light passes through the first filter.
- Mixing up polarisation with diffraction or refraction.