Interactive practical

Intensity variation with polarisation

Measure background light, find the analyser's maximum, then read a light meter every 15° and test Malus' law with a graph of corrected intensity against cos²θ.

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A-Level Physics

Intensity variation with polarisation

WJEC 1420QS — matched to your course.

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Practical focus

Measure transmitted intensity as the relative angle between polarisers changes.

Practical method

Aim

Investigate how the intensity of light transmitted through a polariser and analyser depends on the angle between them (Malus' law, I = I₀cos²θ).

Variables

Independent variable: angle θ between polariser and analyser
Dependent variable: transmitted intensity
Control variables: light source and distance; background light.

Apparatus

Lamp or LED
Two polarising filters, one on a rotating mount with a protractor
Light-dependent resistor or light sensor with data logger
Dark cloth or box

Method

Set the polariser and analyser parallel (θ = 0°) so transmission is maximum, and record the sensor reading.
Rotate the analyser in 15° steps to 180°, recording the intensity each time.
Subtract the background reading taken with the source switched off.
Plot intensity against cos²θ: a straight line through the origin confirms I = I₀cos²θ.

Sample results

Readings corrected for background; I₀ = 100 units.

θ / °	cos²θ	I / units
0	1.00	100
30	0.75	74
45	0.50	51
60	0.25	26
90	0.00	1

Analysis

Intensity against cos²θ is a straight line through the origin with gradient ≈ 100 units, equal to I₀. The light is minimum at 90° (crossed filters), showing light is a transverse wave.

Uncertainty

The protractor reads to ±1°; near 45° this changes cos²θ by about 0.017 (3%). Stray light gives a background of a few units, so subtract it and work in a dark box.

Errors and improvements

Problem	Effect	Improvement
Background light	Intensities too high, especially near 90°	Measure and subtract background; use a dark box
Zero of the protractor misaligned with maximum transmission	All angles shifted	Set θ = 0 at the measured maximum
Non-linear sensor	Graph curves	Calibrate or use a linear light sensor

Risk assessment

Hazard	Risk	Control measure
Hot lamp	Burns	Allow to cool; do not touch
Working in dim light	Trips	Keep the area clear

Common mistakes

Plotting I against θ and expecting a straight line.
Forgetting to subtract background.
Using cos θ instead of cos²θ.

Worked analysis

With I₀ = 100 units at θ = 60°, I = 100 × cos²60° = 100 × 0.25 = 25 units; the measured 26 units agrees within uncertainty.

Common misconception

Crossed polarisers block light because each transmits only one plane of oscillation; this is evidence that light is transverse.

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