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Waves come in two families set apart by the their particles oscillate relative to the way energy travels. That single difference decides whether a wave can be — a property with real uses from photography to radio, and one AQA loves to test.
What you'll be able to do
In a the particles oscillate at to the direction in which the wave transfers energy. A wave sent along a rope, ripples on water, and all electromagnetic waves (including light) are transverse.
Because the oscillations are perpendicular to travel, they can point in any direction within that plane — up-down, left-right, or anywhere between. This freedom is exactly what makes polarisation possible.
Tip — All electromagnetic waves are transverse and travel at m s⁻¹ in a vacuum — a fact worth memorising.
In a the particles oscillate to the direction of energy transfer. Sound is the classic example.
These oscillations create regions where particles are pushed together — — and regions where they are spread apart — . One wavelength runs from the centre of one compression to the centre of the next. Longitudinal waves need a medium, so unlike light, sound cannot travel through a vacuum.
Tip — Because the oscillation is along the line of travel, a longitudinal wave has no “sideways” plane to restrict — so it can never be polarised.
A normal transverse wave, such as light from a bulb, oscillates in planes perpendicular to its travel — it is . A wave oscillates in only one of those planes.
A (Polaroid) only lets through oscillations in one direction, so unpolarised light emerges plane-polarised. Put a second filter at to the first (crossed filters) and no light gets through at all — proof that light is transverse. Longitudinal waves show no such effect, which is how we know sound is longitudinal.
cut out glare: light reflected off water or glass is partially polarised, so a filter aligned to block it reduces the reflected brightness.
must be aligned with the plane of polarisation of the broadcast signal to receive it strongly — a vertically polarised transmission needs a vertical aerial. shines polarised light through transparent plastic models to reveal where stresses concentrate as coloured fringes.
Tip — If a question mentions rotating an aerial or filter for maximum/minimum signal, it is testing polarisation — align to receive, cross to block.
Equation recap
Common mistakes to avoid
Key takeaways
Test yourself
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