There is a crystal through which when light passes through it gets polarised...
Polarization
Light waves from the sun, or even from an artificial light source such as a light bulb, vibrate and radiate outward in all directions. Whether the light is transmitted, reflected, scattered or refracted, when its vibrations are aligned into one or more planes of direction, the light is said to be polarized. Polarization can occur either naturally or artificially. You can see an example of natural polarization every time you look at a lake. The reflected glare off the surface is the light that does not make it through the "filter" of the water, and is the reason why you often cannot see anything below the surface, even when the water is very clear.
Depending on the orientation of the electric field, polarized light can be divided into three types-
Find out more
https://www.quora.com/What-is-polarization-of-light
https://www.physicsclassroom.com/class/light/Lesson-1/Polarization