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Physics 11–12 · Year 12

Malus's law with two polarising filters and a phone light meter (syllabus practical)

Module 7: The Nature of Light (Light: Wave Model)

Practical, model not builtLow risk

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The idea

Light is a transverse wave whose electric field can be filtered to one direction, and the transmitted intensity through a second filter falls with the square of the cosine of the angle between them.

What you need

  • Two sheets of linear polarising film with a protractor pinned through the centre of one, a white LED lamp
  • Phone light-meter app or lux meter, ruler, darkened room

How to do it

  1. Fix the lamp, the first filter and the meter in line; record the reading with the second filter at 0 degrees (maximum).
  2. Rotate the second filter in 15 degree steps to 180 degrees, recording the reading each time; subtract the background.
  3. Plot intensity against angle and against cos^2 of the angle.
  4. Extension: look at the sky at 90 degrees from the Sun and at reflections from a benchtop through one filter as it rotates.

What you should see

Intensity falls smoothly from the maximum at 0 degrees to near zero at 90 degrees and returns at 180 degrees: at 45 degrees it is half the maximum and at 60 degrees one quarter. Intensity against cos^2 is a straight line through the origin with a small residual from the filters' imperfect extinction. Skylight and glare from the bench dim strongly at one filter angle, showing they are partly polarised.

What changes

What you change
angle between the filters' axes
What you measure
transmitted intensity
What you keep the same
  • lamp brightness
  • meter distance
  • background light

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Two crossed filters stop light because they are too thick; they stop it because no field component survives both axes.
  • Rotating one filter by 45 degrees halves the field; it halves the intensity, the field falls by cos 45.

Safety card

Low riskLearners carry it out

Hazards

  • never view the Sun directly, even through filters

Controls

  • use the lamp; view sky away from the Sun

Note

Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

  • Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH12-14PH11/12-3PH11/12-4
  • Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-12-03PY-12WS-04
  • Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

  1. www.nsw.gov.au/sites/default/files/noindex/2025-03/physics-stage-6-syllabus-2017.docx
  2. education.nsw.gov.au/content/dam/main-education/teaching-and-learning/curriculum/key-learning-areas/science/s-6/physics/12Physics_-module-7-guide.docx
  3. phyphox.org
  4. curriculum.nsw.edu.au/learning-areas/science/physics-11-12-2025/outcomes

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