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A wave oscillates in only one plane. Ordinary light oscillates in every direction perpendicular to its travel, but passing it through a polarising filter selects just one. Because only transverse waves can be polarised, this simple effect is decisive evidence about what light is.
What you'll be able to do
In light the electric field oscillates in all directions perpendicular to the direction of travel, changing rapidly and randomly.
A transmits only the component of oscillation along its transmission axis. What emerges is : its oscillation is confined to one plane, containing the direction of travel.
An ideal filter transmits half the intensity of unpolarised light, since on average half the oscillation lies along its axis.
Pass polarised light through a second filter — the — and rotate it. When its axis is parallel to the light’s plane, maximum intensity passes. As it rotates, the transmitted intensity falls.
When the two axes are at — — no light passes at all, because the analyser blocks exactly the plane the first filter let through.
Rotating the analyser through a full turn produces two maxima and two minima, at intervals.
Tip — Intensity follows , not , because intensity depends on amplitude squared and only the component of amplitude gets through.
A longitudinal wave oscillates along its direction of travel — there is only one possible oscillation direction, so there is nothing for a filter to select between. Longitudinal waves cannot be polarised.
Light can be polarised, and crossed polarisers can extinguish it completely. That is only possible if its oscillation is perpendicular to its travel, so light must be a transverse wave.
Sound cannot be polarised, consistent with it being longitudinal. Microwaves can be polarised by a metal grille, confirming they are transverse too.
Tip — In an explanation, name the property that longitudinal waves lack: a choice of oscillation direction perpendicular to travel.
Light from water, glass or a road surface is partially polarised, mostly horizontally. with a vertical transmission axis block much of that reflected glare while still transmitting light from other sources.
: many transparent plastics rotate the plane of polarised light by an amount that depends on internal stress. Viewed between crossed polarisers, a stressed model shows coloured fringes, concentrated where stress is greatest — letting engineers find weak points before building.
place a layer of liquid crystal between crossed polarisers. An applied voltage changes whether the crystal rotates the plane of polarisation, switching each pixel between light and dark.
Photographers use polarising filters to darken skies and remove reflections; broadcasters polarise radio and TV signals, which is why aerials must be aligned.
Tip — Stress-analysis answers should mention crossed polarisers, the plastic rotating the plane of polarisation, and fringes being closest where stress is highest.
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