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.