Malus's Law
Shine unpolarised light through a polariser and a rotating analyser and watch the transmitted intensity trace out cos-squared of the angle between them — then insert a third polariser between two crossed ones and watch light that should be fully blocked pass through again.
What you can adjust
- Source intensity (I0)
- 1 – 10 W/m^2
- Analyser angle
- 0 – 180 deg
- Insert middle polariser
- 0 – 1
- Middle polariser angle
- 0 – 180 deg
Intensity of the unpolarised light source before it reaches the first polariser. Every downstream intensity scales linearly with this — only the RATIO to this value is what Malus's Law actually predicts.
The final polariser's (the analyser's) transmission axis, measured from the first polariser's axis. Marked as a highlight point on the traced curve. 90 degrees (crossed with the first polariser) blocks everything — unless the middle polariser below is switched on.
Insert a third polariser BETWEEN the first polariser and the analyser. With the analyser crossed at 90 degrees, this is the counter-intuitive result: adding a THIRD polariser lets light back through instead of blocking it further.
The middle polariser's transmission axis, measured from the first polariser's axis. Only has an effect when 'Insert middle polariser' is on. 45 degrees, halfway between two 90-degree-crossed outer polarisers, lets the most light through.