In 1887, the German scientist Hertz discovered the influence of light on electric discharge. Studying the spark discharge, Hertz found that if you illuminate the negative electrode with ultraviolet rays, then the discharge occurs at a lower voltage on the electrodes.
It was further discovered that when the light of an electric arc illuminates a negatively charged metal plate connected to an electroscope, the arrow of the electroscope is lowered. This testified to the fact that a metal plate illuminated by an electric arc loses its negative charge. A metal plate does not lose a positive charge under lighting.
The loss of negative electric charge by metallic bodies when they are illuminated by light rays is called the photoelectric effect or simply the photoelectric effect.
The physics of this phenomenon has been studied since 1888 by the famous Russian scientist A. G. Stoletov.
Stoletov studied the photoelectric effect with the help of an installation consisting of two small disks. A solid zinc plate and a thin mesh were mounted vertically against each other, forming a capacitor. His plates were connected to the poles of the current source, and then illuminated by the light of an electric arc.
Light freely penetrated through the grid onto the surface of a solid zinc disk.
Stoletov found that if the zinc plate of the capacitor is connected to the negative pole of the voltage source (it is the cathode), then the galvanometer included in the circuit shows the current. If the cathode is a grid, then there is no current. This means that the illuminated zinc plate emits negatively charged particles, which determine the existence of current in the gap between it and the grid.
Studying the photoelectric effect, the physics of which has not yet been discovered, Stoletov took for his experiments disks from a wide variety of metals: aluminum, copper, zinc, silver, nickel. Attaching them to the negative pole of the voltage source, he observed how an electric current appeared in the circuit of his experimental setup under the influence of an arc. Such a current is called a photocurrent.
With an increase in voltage between the capacitor plates, the photocurrent increased, reaching at some voltage its maximum value, called the saturation photocurrent.
Studying the photoelectric effect, the physics of which is inextricably linked with the dependence of the saturation photocurrent on the magnitude of the light flux incident on the cathode plate, Stoletov established the following law: the magnitude of the saturation photocurrent will be directly proportional to the light flux incident on the metal plate.
This law is called Stoletov.
It was further established that the photocurrent is an electron flux torn by light from a metal.
The theory of the photoelectric effect has found wide practical application. So devices were created, which are based on this phenomenon. They are called photocells.
A light-sensitive layer - the cathode - covers almost the entire inner surface of the glass container, with the exception of a small window for access of light. The anode is a wire ring mounted inside the cylinder. There is a vacuum in the cylinder.
If you connect the ring to the positive pole of the battery, and the photosensitive layer of metal through a galvanometer with its negative pole, then when the layer is illuminated with an appropriate light source, a current will appear in the circuit.
You can turn off the battery completely, but even then we will observe a current that is only very weak, since only a tiny fraction of the electrons emitted by the light will fall on the wire ring - the anode. To enhance the effect, a voltage of about 80-100 volts is needed.
The photoelectric effect, the physics of which is used in such elements, can be observed using any metal. However, most of them, such as copper, iron, platinum, tungsten, are sensitive only to ultraviolet rays. Alkali metals alone - potassium, sodium, and especially cesium - are also sensitive to visible rays. They are also used for the manufacture of cathodes of solar cells.