Liquid crystal is an organic compound composed of long rod-shaped molecules. In the natural state, the long axes of these rod-shaped molecules are substantially parallel.
The first feature of the LCD screen is that the liquid crystal must be poured into two planes with fine grooves to work properly. The grooves on the two planes are perpendicular to each other (90 degrees intersect), that is, if the molecules in one plane are arranged in the north-south direction, the molecules on the other plane are arranged east-west, and the molecules located between the two planes are Forced into a 90 degree twist state. Since the light propagates in the direction in which the molecules are arranged, the light is also twisted by 90 degrees as it passes through the liquid crystal. But when a voltage is applied to the liquid crystal, the molecules are vertically aligned so that the light can be directed out without any twisting.
The second characteristic of the LCD screen is that it relies on the polarizing filter and the light itself. Natural light is randomly scattered in all directions. The polarizing filter is actually a series of increasingly thin parallel lines. These lines form a net that blocks all rays that are not parallel to these lines, and the lines of the polarizing filter are exactly perpendicular to the first one, so that the polarized light is completely blocked. Only when the lines of the two filters are completely parallel, or the light itself has been twisted to match the second polarizing filter, the light is penetrated.
The LCD screen is composed of such two mutually perpendicular polarizing filters, so that all attempts to penetrate light should be blocked under normal conditions. However, since the two filters are filled with twisted liquid crystals, after the light passes through the first filter, it is twisted by the liquid crystal molecules by 90 degrees, and finally passes through the second filter. On the other hand, if a voltage is applied to the liquid crystal, the molecules are rearranged and completely parallel, so that the light is no longer twisted, so it is just blocked by the second filter. In short, power is applied to block light, and light is emitted without power. Of course, it is also possible to change the arrangement of the liquid crystals in the LCD so that the light is emitted when it is powered on, and is blocked when it is not powered. But since the LCD screen is almost always on, only "power up will block the light.