What is a polarizer?

From high school physics, we have learned that light is a wave. The direction of travel of light waves is perpendicular to the electric field and magnetic field. At the same time, the electric field and magnetic field components of the light wave itself are also perpendicular to each other. In other words, the direction of travel and the electric field and magnetic field components are parallel to each other. The role of the polarizing plate is like a fence. It will block the components perpendicular to the fence and only allow the components parallel to the fence to pass through. So if we pick up a polarizing plate and look at the light source, it will feel like wearing sunglasses, and the light becomes darker. But if you stack two polarizing plates together, it will be different. When you rotate the relative angle of the two polarizing plates, you will find that the brightness of the light will become darker and darker as the relative angle changes. When the fence angles of the two polarizing plates are perpendicular to each other, the light cannot pass through at all. Liquid crystal displays make use of this property. Liquid crystal is filled between two polarizing plates that are perpendicular to each other. The electric field is used to control the rotation of the liquid crystal to change the direction of light. In this way, different electric field sizes will form different grayscale brightness.