The photoelectric effect is one of the amazing physical phenomena, the time scales of the development of ideas about which span about three centuries. In this article, we consider the history of this phenomenon, and also give and describe the main equation of the photoelectric effect.
Background of the discovery of the phenomenon
The prerequisites for discovering the phenomenon of the photoelectric effect originated in the distant XVII century, when Isaac Newton advanced the corpuscular theory of light. According to her, the beam of light consisted of multi-colored small particles - corpuscles. This theory lasted a century and a half and successfully explained the phenomena of reflection and refraction of light.
But then the first half of the XIX century came, and the English scientist Thomas Jung in his experiment with a slit and a monochromatic beam of light showed that the physical object in question has a wave nature.
In the 60s of the XIX century, James Maxwell based on theoretical calculations built a fairly coherent theory of electromagnetism, in which he was able to combine all the known phenomena of a magnetic and electrical nature at that time. Maxwell predicted the existence of electromagnetic waves, thereby confirming Jung's experiments.
The German physicist Heinrich Hertz performed experiments in which he proved the existence of the waves predicted by Maxwell and simultaneously discovered the phenomenon of the photoelectric effect.
The experiments of Heinrich Hertz
The idea of ​​setting up Hertz’s experiments was born directly from Maxwell’s theory, which said that an alternating electric or magnetic field is capable of generating electromagnetic waves. The latter are able to induce alternating current in any conductor that receives them.
In 1887, Hertz, using a Rumkorf coil, charged two metal spheres, causing a spark discharge between them. This discharge created a wave which, generating alternating current in the receiver, led to another spark discharge in a small air gap. This discharge was so weak that Hertz placed the receiver in a dark room to see a spark. And here the scientist noticed one strange thing: the spark in the dark room was weaker than in the bright one.
Having published his work, Hertz could not explain the noted changes in the intensity of the spark. A satisfactory explanation was given only in 1905 by Albert Einstein. But before this happened, another significant figure appeared in the history of the discovery of the photoelectric effect.
Alexander Stoletov and his experiments
A. G. Stoletov is an outstanding Russian scientist of the second half of the 19th century, who made a significant contribution to the development of ideas about electromagnetism. But Stoletov’s experiments on the study of the photoelectric effect are best known.
He set these experiments in 1888. They consisted of the following: by connecting an air capacitor to a weak power source, the scientist directed the light from a mercury lamp to the cathode (zinc plate of the capacitor), while he observed the appearance of an electric current in the circuit.
These experiments allowed Stoletov to formulate the first law of the photoelectric effect: the current induced in the circuit is directly proportional to the intensity of the incident light on the cathode. The Russian scientist explained this phenomenon by pulling out negatively charged particles by an electromagnetic wave from the cathode material. We note that at the time of the formulation of these experiments, the electron was not yet open.
Albert Einstein and the modern theory of the photoelectric effect
In 1905, using the results of studies by various scientists (Stoletov, Thomson, Planck), Einstein published an article "On a heuristic point of view regarding the emergence and transformation of light," in which he gave an exhaustive explanation of the phenomenon and presented the photoelectric effect equation.
Modern laws of the photoelectric effect are formulated as follows:
- There is direct proportionality between the light intensity and the induced photocurrent (Stoletov's law).
- There is a certain frequency of light, called the threshold, below which the phenomenon in question is not observed.
- The kinetic energy of an electron pulled out by a photon is directly proportional to the frequency of the photon and does not depend on the intensity of the light incident on the cathode.
- This effect occurs instantly as soon as light hits the material.
Theory of the photoelectric effect. Einstein's equation
To understand the above provisions for the photoelectric effect, we should consider what happens to an electron in an atom when it is irradiated with light. Einstein's main merit was that he was able to guess that it was not an electromagnetic wave that interacts with the electron, but a quantum of light of a certain energy - a photon. The photon is completely absorbed by the electron, transferring its energy to it. Further, the fate of the electron may be as follows:
- If the transferred energy from the photon is not enough to escape from the atom, then the electron first goes into an excited state, and then returns to the ground state with the emission of photons.
- If the photon energy is greater than the electron work function, then it breaks out of the material and goes into a free state.
The photoelectric effect equation has the form:
h Ă— v = h Ă— v 0 + E k .
Here v is the photon frequency, v 0 is the red border of the photoelectric effect or the threshold frequency below which the phenomenon is not observed, E k is the kinetic energy of a free electron, h is the Planck constant.
The photoelectric effect equation shows that the photon energy (h Ă— v) is spent on tearing an electron out of the material (h Ă— v 0 ) and telling it a certain speed (E k ).
Photo effect and solar panels
The photoelectric effect is widely used to produce electrical energy from sunlight. This energy is consumed both to satisfy domestic needs, and to power electronics on space satellites.
The main material for solar panels is currently silicon. The induced EMF in the battery occurs when light falls on the pn region of the semiconductor junction.