9702 · 7.5
Polarisation flashcards
Revision flashcards for Cambridge 9702 Polarisation (syllabus 7.5). Flip, recall, then mark a real past-paper question.
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What fundamental characteristic must a wave possess to be polarisable?
It must be a transverse wave.
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How does the phenomenon of polarisation provide evidence about the nature of a wave?
It confirms that the wave is transverse, as only transverse waves can be polarised.
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State Malus' Law for calculating the intensity of a polarised wave after passing through a filter.
$I = I₀\cos²θ$
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In Malus' Law ($I = I₀\cos²θ$), what does the variable $θ$ represent?
The angle between the plane of oscillation of the incident polarised wave and the transmission axis of the polarising filter.
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What general effect does a polarising filter have on the intensity of any wave passing through it?
It reduces the wave's intensity.
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Define polarisation.
The restriction of a wave's oscillations to a single plane.
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Why can't longitudinal waves be polarised?
Their oscillations are inherently parallel to their direction of energy transfer, so they already vibrate in only one plane.
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What is an unpolarised transverse wave?
A transverse wave oscillating in various planes perpendicular to its direction of energy transfer.
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What is the 'transmission axis' of a polarising filter?
The specific direction/plane that the filter allows wave oscillations to pass through.
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In Malus' Law, what happens to intensity when $θ = 90°$?
Intensity $I = 0$, meaning no light passes through (perfectly blocked).
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In Malus' Law, what happens to intensity when $θ = 0°$?
Intensity $I = I₀$, meaning maximum light passes through (fully transmitted).
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Are electromagnetic waves (like light) polarisable? Why?
Yes, because they are transverse waves.
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An unpolarised light beam of intensity $I_{un}$ passes through an ideal polarising filter. What is the intensity of the transmitted light?
The intensity is halved, becoming $I_{un}/2$.
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What is an 'analyser' in the context of polarisation?
It is a second polarising filter placed after a first one, used to 'analyse' the properties (like plane of polarisation and intensity) of the already polarised light.
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Why do photographers use polarising filters?
To reduce glare and reflections from surfaces like water and glass, and to enhance the colour and contrast of skies by filtering scattered, polarised light.
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What is the role of the two polarising filters in a standard LCD screen?
They are typically crossed (at 90°). The liquid crystal layer in between rotates the plane of polarisation to control whether light passes through the second filter, creating a pixel.