Many people know that Albert Einstein received the Nobel Prize in 1921 for explaining to him the photoelectric effect with the use of ideas about light particles - photons. However, few people know that the first experiments to study this phenomenon were conducted by Russian physicist Alexander G. Stoletov. In this article, we consider what Stoletov’s experiments were and what conclusions they led the scientist.
What inspired Stoletov to conduct his experiments?
To understand where the prerequisites for conducting Stoletov’s experiments on the photoelectric effect came from, one should pay attention to the state of physics at the end of the 19th century. This time is marked by the statement of the wave nature of light, which was formulated by Huygens as opposed to Newton's corpuscular theory in the second half of the 17th century. In particular, Jung’s experiments with a monochromatic beam of light passing through two slits showed that light is a wave (observation of the phenomena of interference and diffraction).
Further, in the 1860s, Maxwell, thanks to his theoretical work, showed that electricity, magnetism and light are phenomena of the same electromagnetic nature. To prove this in practice, a German scientist, Heinrich Hertz, conducted a series of experiments starting in 1885. Hertz, trying to prove Maxwell's theoretical calculations, unknowingly discovered the photoelectric effect in 1887.
Hertz's experiments
According to some sources, even before the experiments of Hertz, Stoletov began to deal with the photoelectric effect, since evidence of its existence already existed by 1887 (in the mid-19th century, Willoughby Smith discovered the phenomenon of the dependence of the electrical conductivity of selenium semiconductor on illumination). However, at present, the fact of the discovery of the photoeffect is attributed precisely to Hertz. Consider his experiences.
Hertz's experiments are extremely simple in their idea: the scientist believed that if you charge two metal spheres with opposite charges in sign and bring them close to each other, a spark discharge will occur. According to Maxwell, this discharge should lead to the generation of an electromagnetic wave. The latter, in turn, will excite an alternating electric current in any closed conductor, and if the contact break in this conductor has a small gap, then it will be possible to observe the spark induced by the electromagnetic wave in this gap.
After conducting the described experiment, Hertz confirmed Maxwell's theory, but he noticed one strange effect that he could not explain. The spark induced in the receiver was very intense when light fell on the gap. The increase in conductivity in the air gap due to light is called the photoelectric effect.
The first experience of A. G. Stoletov
Unlike Hertz, Stoletov purposefully studied the phenomenon of the photoelectric effect. In this paragraph of the article, we briefly describe the experience of Stoletov, which he conducted in 1888 (a universally recognized date).
For his experiment, the scientist used an air condenser consisting of a grating and a zinc plate. A potential difference was applied to this capacitor, the zinc plate being the cathode and the lattice an anode. The circuit closed, and a galvanometer was included in it. Naturally, he did not show any current, since the air gap of the capacitor is a good insulator. Then the scientist took a mercury lamp and illuminated it through a grating a plate of zinc. As soon as he did this, the galvanometer instantly began to show that current was flowing along the circuit. When the lamp was removed, the current in the circuit stopped.
When Stoletov changed the poles on the plates of the capacitor, that is, the lattice became the cathode, and the zinc plate became the anode, even when the lamp was illuminated, there was no current in the circuit.
The second experiment of Stoletov
Another important experiment of Stoletov for understanding the photoelectric effect was as follows: the scientist took an electroscope, positively charged it, and then irradiated it with the light of a mercury lamp. No effect was observed, and the petals of the electroscope remained raised up, indicating the existence of a charge on the device. When Stoletov changed the sign of the charge, then when lighting it quickly drained from the device, and the petals fell.
The laws of Stoletov
The interpretation of the experiments led to the formulation of two of the four modern laws of the photoelectric effect by Stoletov.
Since a positively charged electroscope did not respond to light (second experiment), Stoletov came to the conclusion that somehow an electromagnetic wave pulls a negative charge out of the material (it is now known that this charge carries away electrons).
In addition, the Russian scientist established a directly proportional relationship between the photocurrent and the light intensity of the lamp (the first law of the photoelectric effect or Stoletov’s law).
Finally, the scientist observed that the photoelectric effect occurs without delay, that is, instantly, as soon as light hits the cathode (now this position is known as the 4th law of the photoelectric effect).
Modern understanding of the photoelectric effect
Thanks to the experiments of Stoletov, as well as subsequent experiments by Thomson with cathode rays, in 1905, Albert Einstein was able to explain the photoelectric effect from the point of view of the physics of processes. Using Planck's idea of the quantization of light flux, Einstein suggested the following: when a photon (quantum of electromagnetic radiation) falls on a material, its energy is completely absorbed by the electrons of the latter. If this energy is greater than a certain value (electron work function), then a charged particle breaks out of the material. In this way, the material receives a positive charge, and the medium around it becomes conductive due to the presence of free electrons in it.
The most striking example of the modern use of this effect is the generation of electrical energy using solar panels.