In simple terms
A friendly intro before the formal notes — no formulas yet.
Polarisation
Cambridge 9702 Paper 2 — Polarisation (7.5). Senpai Corner diagram-backed pilot with premium structure and live visuals.
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7.5 Polarisation.
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Only transverse waves can be polarized.
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Polarization means that vibrations are restricted to one direction.
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Waves can be polarised through a polariser or polarising filter.
What this topic covers
The official Cambridge syllabus points this lesson works through.
- 7.5.1
Understand that polarisation is a phenomenon associated with transverse waves
- 7.5.2
Recall and use Malus's law () to calculate the intensity of a plane-polarised electromagnetic wave after transmission through a polarising filter or a series of polarising filters (calculation of the effect of a polarising filter on the intensity of an unpolarised wave is not required)
Explore the concept
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Key formulas
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Full topic notes
Formal explanation with the rigour you need for the exam.
Unveiling Wave Vibrations
Waves transfer energy through oscillations. For transverse waves, these oscillations are perpendicular to the direction the energy travels. Think of ripples on water or a wave on a string. In contrast, for longitudinal waves, like sound, the oscillations are parallel to the energy transfer, compressing and expanding the medium. This fundamental difference is what makes polarisation possible for some waves, but not others.
The Transverse Test: Only Transverse Waves Can Be Polarised
Polarisation is the ultimate experimental test for distinguishing between transverse and longitudinal waves. Only transverse waves can be polarised because their oscillations occur across many possible planes perpendicular to their travel direction. Longitudinal waves, however, naturally oscillate only back and forth along their path of travel, meaning their vibrations are already restricted to a single direction and cannot be further 'polarised'.
7.5 Polarisation.
Only transverse waves can be polarized.
Polarization means that vibrations are restricted to one direction.
Waves can be polarised through a polariser or polarising filter.
Example polaroid sunglasses.
Polarising a wave reduces its amplitude.
How Polarising Filters Work
To take an unpolarised transverse wave (which oscillates in many planes) and make it vibrate in just one, we use a special device called a polarising filter (or simply a polariser). This filter has a specific transmission axis. It acts like a very fine grid or slit, only allowing wave components that oscillate parallel to its axis to pass through. All other oscillation components are either absorbed or reflected.
When an unpolarised wave first passes through an ideal polarising filter, its intensity is always reduced by half, because on average, half of its oscillation components are blocked. If a polarised wave then passes through a second filter (often called an analyser), the final intensity depends on the precise angle between the incoming wave's plane of oscillation and the analyser's transmission axis.
Malus' Law: Calculating Intensity
Malus' Law is a critical formula used to calculate the intensity () of a plane-polarised wave after it passes through a polarising filter (or analyser). In this formula, represents the initial intensity of the plane-polarised wave incident on the filter. The angle (theta) is the angle between the original plane of oscillation of the incident polarised wave and the transmission axis of the polarising filter.
Understanding the extreme cases of Malus' Law is vital:
If : The wave's oscillation plane is perfectly aligned with the filter's transmission axis. Since , then . So, , meaning maximum transmission.
If : The wave's oscillation plane is perpendicular to the filter's transmission axis. Since , then . So, , meaning zero transmission (the wave is completely blocked).
Real-World Applications of Polarisation
Polarisation is not just a lab curiosity; it is the principle behind many technologies we use daily. Understanding how it's applied helps to solidify the concept.
Polaroid Sunglasses: Reduce glare from horizontal surfaces (like water or roads) by using vertically aligned polarising filters. This blocks the horizontally polarised light that makes up most of the reflected glare.
LCD Screens: Liquid Crystal Displays (in TVs, monitors, calculators) use two polarising filters, usually at 90° to each other ('crossed'). A liquid crystal layer between them can rotate the plane of polarisation of light when a voltage is applied, thus controlling whether light passes through the second filter to light up a pixel.
3D Cinema: Some 3D movie systems use glasses with differently polarised lenses (e.g., clockwise and anti-clockwise circular polarisation). Two projectors superimpose images with different polarisations onto the screen. Each lens only allows one image to pass to each eye, which the brain then combines to create a perception of depth.
Stress Analysis: Engineers view transparent plastic models of structures (like bridges or machine parts) between two crossed polarisers. When the model is stressed, it alters the polarisation of light passing through it, revealing stress concentration points as coloured fringes.
Photography: Photographers use polarising filters on camera lenses to reduce unwanted reflections, darken blue skies for better contrast, and saturate colours by filtering out polarised scattered light.
Worked examples
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A plane-polarised light wave with an initial intensity of passes through a polarising filter. If the angle between the light's plane of oscillation and the filter's transmission axis is , what is the intensity of the light after passing through the filter?
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Identify knowns:
Unpolarised light of intensity 50 W m⁻² is incident on a polarising filter (Polariser A). The transmitted light then passes through a second filter (Analyser B), whose transmission axis is at an angle of 60° to the axis of Polariser A. Calculate the final intensity of the light emerging from Analyser B.
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Intensity after Polariser A:
How it all connects
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Glossary
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Quick check
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Revision flashcards
Flip the card. Test yourself before the exam.
What fundamental characteristic must a wave possess to be polarisable?
It must be a transverse wave.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
- ✓
7.5 Polarisation.
- ✓
Only transverse waves can be polarized.
- ✓
Polarization means that vibrations are restricted to one direction.
- ✓
Waves can be polarised through a polariser or polarising filter.
- ✓
Example polaroid sunglasses.
- ✓
Polarising a wave reduces its amplitude.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
9702/23 · Q2(b)(iii)
Use your answers in (b)(i) and (b)(ii) to calculate θ.
Extra simulations & links
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Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do 9702/23 · Q2(b)(iii) on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.