Malus's Law: Two Polarizers at an Angle

1 · Predict

Light passes through one polarizer, then a second one at some angle to the first. Does the transmitted intensity depend smoothly on the angle between the two polarizers' axes?

• Yes — transmitted intensity follows cos² of the angle between the axes, reaching zero at exactly 90° ('crossed' polarizers).
• Transmitted intensity decreases linearly with the angle.
• Light either passes completely or is completely blocked — no smooth transition.

2 · Set Up

1. Open the crossed-polarizers preset and press Reset. The two polarizers' axes are set 90° apart (fully crossed — no light transmitted).
2. Enable the transmitted-intensity readout.
3. Set the angle between the two polarizers' axes for each trial and record the fraction of light transmitted through the second one.

3 · Collect Data

Plot transmitted fraction (y-axis) against Δθ (x-axis) for your three trials. Where does it hit exactly zero?

4 · Analyze

1. For one trial, compute the fraction transmitted through the second polarizer as cos²(Δθ). Compare to the table.
2. Explain why the fraction transmitted through P2 depends only on the ANGLE BETWEEN the two axes, not on their absolute orientation — and why exactly 90° ('crossed') gives zero transmission.

5 · Extend

1. Polarized sunglasses block horizontally-polarized glare (like reflections off water or roads) by orienting their polarizing axis vertically. Explain, using Malus's law, why tilting your head sideways while wearing polarized sunglasses can make glare suddenly reappear.
2. LCD screens use crossed polarizers with a liquid crystal layer between them that can rotate light's polarization on command, controlling how much light gets through each pixel. Explain why 'crossed' (rather than parallel) polarizers are the natural default 'off' state for a pixel.

The Physics Behind This Experiment

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