Everyone knows that light is also a kind of wave, and the traveling direction of light waves is perpendicular to the electric and magnetic fields. At the same time, the electric and magnetic field components of the light waves are also perpendicular to each other. That is to say, the direction of travel and the electric and magnetic field components are parallel to each other. The polarizer acts like a fence, blocking the component perpendicular to the fence and only allowing the component parallel to the fence to pass. So if we pick up a polarizer and look at the light source, it will feel like wearing sunglasses, and the light will become darker. But if you stack two polarizers together, it will be different.
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When rotating the relative angle of the two polarizing plates, you will find that with the relative angle, the brightness of the light will become darker and darker. When the fence angles of the two polarizing plates are perpendicular to each other, the light cannot pass through at all. The liquid crystal display is accomplished by using this feature. Use the upper and lower fences between the vertical polarizers to fill the liquid crystal, and then use the electric field to control the rotation of the liquid crystal to change the direction of light. In this way, different electric field sizes will result in different grayscale brightness.