As we know, the state of matter that people familiar with is gas, liquid, solid, ion and liquid crystal. Liquid crystal is the combination of molecules with special structure. It is liquid in a certain temperature range and crystal in a lower temperature. It not only has special physical property of flowing, but also has special photoelectric characteristic of crystallizing. The composition of liquid crystal is a kind of organic compound whose center is a carbon molecule. The liquid crystal which has two kinds of substances at the same time is the combination of the intermolecular force. It not only has special optical properties, but also is very sensitive to electromagnetic field, so it has very practical value and it is widely used in displaying field since the middle of 20th century.
The common applications of liquid crystal are display panels of digital watches and calculators. The working principle of liquid crystal is photoelectric effect that electrical signal is transforming into optical signal. It is transparent in normal conditions that the molecules of liquid crystal are orderly and systematic. You can see a picture in which liquid crystal molecules are stimulating by electricity, some of them become non-transparent while some darker and some lighter. People produce thermometers and poison gas alarms according to the discoloring characteristic of liquid crystal. For example: liquid crystal thermometers will turn red, green or blue with the change of temperature.