Lesson summary
7.5.1 Polarisation — AS PHYSICS 9702 notes: Explain polarisation of transverse waves, the action of polarising filters and use Malus’s law.Transverse waves — they oscillate with displacement perpendicular to their direction of travel, in any plane perpendicular to propagation.Ele…
- Electromagnetic wave
- Polarised wave
- Vertically or horizontally polarised
- Transmission
- Blocked waves
- Perpendicular axis
Section 1 of 8Polarisation of transverse waves
Polarisation of transverse waves
Transverse waves
- Transverse waves they oscillate with displacement perpendicular to their direction of travel, in any plane perpendicular to propagation.
Electromagnetic wave
- Electromagnetic wave it oscillates in more than one plane, with a magnetic field on one plane and an electric field on another.
- Both planes are at right angles to each other and to the direction of wave motion.
Polarised wave
- Polarised wave vibrations are restricted to one plane of oscillation, still perpendicular to the direction of energy transfer.
Unpolarised wave
- Unpolarised wave it oscillates in many planes.
Vertically or horizontally polarised
- Vertically or horizontally polarised it oscillates only in the vertical or horizontal plane.
Section 2 of 8Polarising filters
Polariser
- Polariser an optical layer or lens with many parallel tiny slits.
Transmission
- Transmission it only allows waves vibrating parallel to the slits to pass through.
Blocked waves
- Blocked waves waves not oriented in the correct plane are blocked.
Perpendicular axis
- Perpendicular axis a polarised wave meeting a filter with a transmission axis perpendicular to it is not transmitted at all.
Which waves
- Which waves only transverse waves can be polarised, for example radio waves passing through a metal grid.
Not polarised
- Not polarised sound waves, ultrasound and diffracted water waves.
Section 3 of 8Polaroid sunglasses
Lenses
- Lenses they contain polarising filters with vertically oriented transmission axes.
Effect
- Effect they block horizontally polarised light, reducing glare.
Section 4 of 8Intensity after a polariser
Half rule
- Half rule when unpolarised light of intensity I0 passes through a polariser, the transmitted intensity is I = I0/2.
Analyser
- Analyser a second polarising filter placed after the first.
Same orientation
- Same orientation if the polariser and analyser have parallel transmission axes, the intensity is unchanged.
Section 5 of 8Malus’s law
Statement
- Statement if the analyser is rotated by angle θ to the polariser, the transmitted intensity is I = I0 cos2θ.
- I is the transmitted intensity in W m−2.
- I0 is the maximum intensity of the already polarised light.
θ
0° or 180°
- cos2θ = 1, so the intensity is I0, the maximum.
θ
90° or 270°
- cos2θ = 0, so no light is transmitted and all waves are absorbed.
Unpolarised source
- Unpolarised source first apply the half rule, then use Malus’s law with the polarised intensity as I0.
Section 6 of 8More from the specification
1Polaroid sunglasses glare reduction
- Polaroid sunglasses glare reduction polaroid sunglasses contain vertically aligned polarizing filters that absorb horizontally polarised glare from reflective water surfaces.
2Malus's law for incident polarised light
- Malus's law for incident polarised light transmitted intensity of light incident upon an analyser varies with cosine squared of the transmission axis angle.
3Transverse wave polarisation conditions
- Transverse wave polarisation conditions polarisation happens exclusively in transverse waves whose oscillating field vectors vibrate perpendicular to energy propagation pathways.
4Plane polarization of transverse waves
- In plane-polarized electromagnetic waves, electric field vector oscillations are confined to a single plane perpendicular to the direction of wave propagation.
5Malus's law for transmission through polarizers
- The transmitted light intensity passing through an analyser rotated at angle theta to the transmission axis of polarized light is I = I0 cos2(theta).
6Polaroid sunglasses and glare reduction
- Glare reflected from flat horizontal surfaces (water, roads) is predominantly horizontally polarized;
- vertical transmission axes in Polaroid sunglasses absorb this glare.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Polarised wave | Transverse wave whose oscillations are restricted to a single plane. |
| Polariser | Filter that transmits only oscillations parallel to its transmission axis. |
| Analyser | Second polarising filter placed after the polariser. |
| Malus’s law | I = I₀ cos²θ for polarised light through an analyser at angle θ. |
| Half rule | Intensity of unpolarised light is halved after passing through a polariser. |
Section 8 of 8Exam technique
1
Forgetting to square the cosine in Malus’s law — I = I₀ cos²θ.
2
Applying Malus’s law to unpolarised light — halve the intensity first.
3
Thinking sound can be polarised — only transverse waves can.
4
Using the angle between orders rather than between transmission axes.