Malus’s law

For linearly polarized light with intensity I₀, the intensity transmitted through an ideal polarizer, I, can be described by Malus's law, where θ is the angle between the incident linear polarization and the polarization axis.

I=I₀cos²θ

For parallel axes (0°), 100% transmission is achieved. For polarizing lenses at 45°, 50% transmission is achieved. Finally, for lenses with a 90° axes, also known as crossed polarizers, there is 0% transmission. In real world applications, the transmission never reaches exactly 0%, therefore, polarizers are characterized by an extinction ratio, which can be used to determine the actual transmission through two crossed polarizers.

Figure: Malus's law explained for linearly polarized light.