Photocell principle is photoelectric effect. One is the photoelectric tube of semiconductor material type. Its working principle: photodiode, also known as photosensitive diode, is a light receiving device made by using the photosensitive characteristics of semiconductor. When the light intensity increases, the minority carrier concentration generated by the intrinsic excitation in the p and n regions on both sides of the pn junction increases. If the diode reverses, the reverse current increases. Therefore, the reverse current of the photodiode increases with the increase of light intensity. Photodiode is a special kind of diode, which works in reverse bias state.

Common semiconductor materials include silicon and germanium. Such as the light switch used in our corridor. Another type of photoelectric tube is the electronic tube type. Its working principle is to make a curved surface of alkali metal (such as potassium, sodium, cesium, etc.) as the cathode, and the other pole is the anode. A positive voltage is applied between the two poles. In this way, when there is light, the alkali metal generates electrons, and a beam of photoelectron current will be formed, so that the two poles will be connected, the light will disappear, the photoelectron flow will also disappear, and the two poles will be disconnected.

Light irradiation on some substances causes changes in the electrical properties of substances. This kind of phenomenon of light transmission is called photoelectric effect. The effect that the metal surface emits electrons under the action of light irradiation. The electrons emitted are called photoelectrons. When the wavelength of light is less than a certain critical value, electrons can be emitted, that is, the limiting wavelength. The corresponding frequency of light is called the limiting frequency. The critical value depends on the metal material, while the energy of emitting electrons depends on the light The wavelength is independent of light intensity, which cannot be explained by the fluctuation of light. Another point that contradicts the volatility of light is the instantaneous nature of the photoelectric effect. According to the volatility theory, if the incident light is weak and the irradiation time is longer, the electrons in the metal can accumulate enough energy and fly out of the metal surface. But the fact is that as long as the frequency of light is higher than the limit frequency of metal, the brightness of light, no matter how strong or weak, is almost instantaneous, and the generation of photons is not more than ten negative ninth power seconds. The correct explanation is that light must be composed of strictly prescribed energy units (i.e. photons or photons) related to wavelength.