Malus's law
Intermediate
Intensity through an analyser.
I = I₀cos²θIntensity through an analyser is I = I₀cos²θ. Two crossed polarisers block all light, but a third at 45° between them lets a fraction through.
Unpolarised light through one polariser halves; then cos²θ.
Initial halving occurs only for unpolarised input.
In more detail
Malus's law gives the intensity of light transmitted through a polariser, I = I0 cos²θ, where θ is the angle between the light's polarisation and the polariser's axis.
Real-world uses
- Polarisers are used to control brightness in displays.
- Measuring polarisation angles in optical experiments.
- Engineers using LCD screens use Malus's law to predict how much light passes through two crossed polarisers at an angle.
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Frequently asked questions
What happens at 90 degrees?
No light is transmitted, because the polarisation is perpendicular to the axis.
Does a polariser reduce intensity for unpolarised light?
Yes, by half on average, because only one direction is passed.
Why does light vanish with crossed polarisers?
The second polariser blocks the light passing through the first, since the field is perpendicular to its axis.
Related topics in Optics
Diffraction grating · Resolving power · Polarisation of light · Brewster's angle · Polarisers and LCDs · Birefringence