By Zencalcy Editorial TeamLast reviewed About us
A polarising filter passes only the component of light aligned with its axis. Rotating the filter changes the transmitted intensity as the square of the cosine of the angle, which is Malus's law.
Transmitted intensity through a polarising filter.
I = I₀ cos² θ
I is transmitted intensity, I₀ the incident intensity and θ the angle between the incident polarisation and the filter axis. At 90° the transmission is ideally zero.
| Symbol | Meaning |
|---|---|
| I | Transmitted intensity. |
| I₀ | Incident intensity before the filter. |
| θ | Angle between polarisation and filter axis. |
I = 1000 × cos² 30° = 750 W/m². At 60° the transmission falls to 250 W/m², and at 90° it reaches zero.
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The first filter transmits half the intensity regardless of its angle. Malus's law then applies to the already polarised beam at any second filter.
Results are computed from the standard equations of classical mechanics. Unit conversions use the SI definitions published by NIST. Air resistance is ignored unless the calculator names a drag model.
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Content updated: September 2026