The liquid crystal of LCD is an organic compound composed of long rod-shaped molecules.
In the natural state, the long axes of these rod-like molecules are roughly parallel.
The first feature of LCD is that liquid crystal must be poured into two planes with fine grooves to work normally. The grooves on these two planes are perpendicular to each other (90 degree intersection)
That is to say, if the molecules on one plane are arranged in the north-south direction, the molecules on the other plane are arranged in the east-west direction, while the molecules located between the two planes are forced into a 90-degree torsion state.
Since the light travels along the direction of the molecules, the light is also twisted 90 degrees when passing through the liquid crystal.
However, when a voltage is applied to the liquid crystal, the molecules will be rearranged vertically so that the light can be directly emitted without any torsion.
The second characteristic of LCD is that it relies on polarized filters and light itself. Natural light is randomly divergent in all directions. Polarized filters are actually a series of thinner and thinner parallel lines.
These lines form a net, blocking all the light that is not parallel to these lines. The line of the polarized filter is exactly perpendicular to the first one, so it can completely block the polarized light.
Only when the lines of the two filters are completely parallel or the light itself has been twisted to match the second polarized filter can the light penetrate.
The LCD screen is made up of two polarized filters that are perpendicular to each other, so under normal circumstances all light that is trying to penetrate should be blocked.
However, since the two filters are filled with twisted liquid crystal, after the light passes through the first filter, it will be twisted 90 degrees by the liquid crystal molecules and finally pass through the second filter.
On the other hand, if a voltage is applied to the liquid crystal, the molecules will be rearranged and completely parallel, so that the light will not be twisted, so it is just blocked by the second filter.
In short, the power is blocked by the power, and the light is emitted without power.
Of course, it is also possible to change the arrangement of the liquid crystal in the LCD so that the light is emitted when it is powered on and blocked when it is not powered on.
However, since the LCD screen is almost always lit, only 'power-on will block the light.