Malus's Law
The law stating that intensity of linearly polarized light transmitted through an analyzer is proportional to the squared cosine of the angle between polarizer and analyzer transmission axes.
Governing formula
I = I0 cos² θ
SI unit
W/m²
In depth
When polarizer and analyzer axes are parallel ($θ = 0^°$), transmission is maximum ($I = I_0$). When crossed at $90^°$, transmitted light intensity drops to zero ($I = 0$).
Examples in the real world
Rotating a secondary polarizing filter in front of a monitor to dim or completely block the screen display.