In the 20s of the XIX century, mankind opened the possibility of converting thermal energy into electrical energy (Seebeck and Peltier experiments). At the end of the same century, another way of generating electricity was discovered - with the help of light. This article is devoted to the question of what is a photoelectric effect.
Who discovered the photo effect when?
The discovery of the photoelectric effect in science has a long history and is directly related to the disputes of scientists about the nature of light. In 1887, conducting experiments to prove the existence of electromagnetic waves, Heinrich Hertz discovered the phenomenon of the photoelectric effect. What is the photoelectric effect, Hertz could not explain, but published the "strange" results. The essence of these results was that the spark induced in the air gap of the receiver had a greater brightness when the receiver was in the light than when the scientist put it in a dark room.
A year later, that is, in 1888, the Russian scientist Alexander Stoletov conducted a series of experiments, from which he made a number of important conclusions regarding the features of the photoelectric effect. Currently, the first law of the photoelectric effect bears his last name.
Who developed the theory of the observed effect?
Albert Einstein did this in 1905, for which he received the Nobel Prize in Physics in 1921.
A modern understanding of the processes occurring during this effect is based entirely on Einstein's ideas. His main merit was the recognition that light is not only a wave, but also exhibits corpuscular properties in interaction with matter (this position is known as wave-particle duality). In particular, an electromagnetic wave is not uniformly distributed in space, but consists of energy bunches (quanta), which were later called photons. When such a photon falls on a material, it interacts with only one electron of any atom, transferring all its energy to it.
In fairness, we note that the corpuscular theory was developed long before Einstein in the distant XVII century, and Isaac Newton did it. Einstein didn’t just revive Newton’s idea, but included Max Planck’s ideas about light quanta with energy h * v.
What is a photo effect?
Now we turn to a direct explanation of the processes at the atomic level associated with the phenomenon under consideration.
By a photoelectric effect is meant the pulling out of electrons from a material and their transfer to a free state due to the light incident on this material. This happens as follows: when a photon hits an atom of matter, it interacts with an electron, transferring all its energy to it. Due to this energy, the electron transfers to higher energy levels of the atom (atom excitation). If the amount of energy transferred is large enough, the electron will be able to break away from the atom and fly out into the interatomic space.
Often talk about the internal and external photo effect. They differ from each other only in where the electron "torn" from the atom gets (if it remains inside the material, then they talk about the inside, but if it flies into the atmosphere, then about the external photoelectric effect). The experiments of Hertz and Stoletov mentioned above are an example of an external photoelectric effect. An example of the internal is the operation of modern solar panels.
Basic laws of the photoelectric effect
Thanks to the experiments of the late 19th century and the theory of the photoelectric effect developed by Einstein at the beginning of the 20th century, the following laws can be formulated for this phenomenon:
- The intensity of the incident radiation and the current strength arising in the circuit are directly dependent.
- There is a certain frequency boundary, the photoelectric effect below which does not occur, that is, photons with a frequency below the threshold cannot “pull” the electrons from the atoms.
- The speed of the emitted electrons does not depend on the intensity of the light incident on the material, but depends on its frequency.
- The photoelectric effect is an instant process (the delay does not exceed 1 ns).
Einstein's equation
Having figured out what a photoelectric effect is, we now give an equation that describes it:
h * v = A + E k .
Here v is the phonon frequency, A is the energy that must be expended in order to “tear” the electron out of the atom, E k is the kinetic energy of the emitted electron. The formula h * v describes the photon energy according to the ideas of Max Planck (h is the Planck constant).
One important thing follows from the Einstein equation: the minimum photon energy, the photoelectric effect at which is still possible, will be equal to the electron work function:
h * v 0 = A (E k = 0).
The frequency v 0 is called the red border for this physical phenomenon. Since the photon frequency is related through the speed of light to its wavelength, the photoelectric effect equation can be rewritten as follows:
h * c / λ 0 = A (E k = 0).
For many metals, the electron work function A ranges from 2 to 6 eV; wavelengths from 580 to 210 nm (part of the visible and ultraviolet spectrum) correspond to these values.
The concept of saturation current
Considering the question of what is the photoelectric effect, we should talk about the saturation current. Let's do the following experiment: take an air condenser formed by two metal plates, connect it to an electric circuit and direct a monochromatic light beam of a certain intensity to the plate cathode. The galvanometer will show that a current has appeared in the circuit. Now we will gradually increase the voltage between the plates of the capacitor, then the current will also increase to a certain value, and then it will become constant, regardless of the voltage. This is shown in the figure below.
Here I const. - saturation current. As can be seen from the graph, some current will exist in the circuit even at zero potential.
The described fact can be explained as follows: when light hits the cathode, then at its constant intensity and frequency, the photoelectric effect leads to the appearance of free electrons. The latter, flying out of the metal, move in arbitrary directions and only some of them will fall on the second plate of the capacitor. When potential is applied to the plates, more and more electrons begin to reach the opposite plate. This increase occurs until all the "torn" electrons are carried away by the electric field, that is, saturation occurs (I const. ).
A further increase in current is possible only due to an increase in light intensity (the number of "torn" electrons grows) or due to an increase in the frequency of light (increases E k electrons).
The concept of inhibitory potential
The above explanation of the processes between the plates of the capacitor allows us to conclude that the current will exist even if you change the sign of the potentials (the plate irradiated with light becomes the anode). As soon as the potential reaches such a value that the "torn out" electrons with the highest energy will return back to the anode without reaching the cathode, then the current in the circuit will stop. This potential is called inhibitory. It is marked in the previous figure by the symbol U 0 .
The braking potential U 0 is independent of the light intensity and increases with increasing photon frequency.