Two crossed polarisers block all light

Wave Types, Polarisation & Stationary Waves — AQA A-Level Physics

When unpolarised light passes through polariser A, it becomes polarised along A's transmission axis. Maximum intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. is transmitted.
If polariser B has its transmission axis parallel to A, all the polarised light passes through B.
As B is rotated, the transmitted intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. varies periodically with angle.
When B's axis is perpendicular to A (90 degrees rotation), no light passes through. The intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. drops to zero.
The intensity varies as a cos-squared function with angle, but AQA does not require Malus's Law calculations.

Examiner Tips and Tricks

You may be asked to describe the intensity variation or draw the graph of intensity vs angle for two polarisers.
The graph has maxima at 0 and 180 degrees and minima (zero) at 90 and 270 degrees.
You will not be asked to do Malus's Law calculations.