1. The history of the discovery of the photoelectric effect
2. Laws of Stoletov
3. Einstein's equation
4. Internal photoelectric effect
5. Application of the phenomenon of photoelectric effect
Introduction
Numerous optical phenomena have been consistently explained on the basis of ideas about the wave nature of light. However, in late XIX- the beginning of the XX century. Phenomena such as the photoelectric effect, X-rays, the Compton effect, the radiation of atoms and molecules, thermal radiation, and others were discovered and studied, the explanation of which from the wave point of view turned out to be impossible. An explanation of the new experimental facts was obtained on the basis of corpuscular ideas about the nature of light. A paradoxical situation has arisen associated with the use of completely opposite physical models of a wave and a particle to explain optical phenomena. In some phenomena, light exhibited wave properties, in others - corpuscular.
Among the various phenomena in which the action of light on matter is manifested, important place takes photoelectric effect, that is, the emission of electrons by a substance under the action of light. The analysis of this phenomenon led to the idea of light quanta and played an extremely important role in the development of modern theoretical concepts. At the same time, the photoelectric effect is used in photocells, which have received exceptionally wide application in the most diverse fields of science and technology and promise even richer prospects.
The history of the discovery of the photoelectric effect
The discovery of the photoelectric effect should be attributed to 1887, when Hertz discovered that illuminating the spark gap electrodes under voltage with ultraviolet light facilitates the spark between them.
The phenomenon discovered by Hertz can be observed in the following easily feasible experiment (Fig. 1).
The value of the spark gap F is selected in such a way that in a circuit consisting of a transformer T and a capacitor C, the spark jumps with difficulty (once or twice per minute). If the electrodes F, made of pure zinc, are illuminated with the light of an Hg mercury lamp, then the discharge of the capacitor is greatly facilitated: a spark begins to jump. 1. Scheme of Hertz's experiment.
The photoelectric effect was explained in 1905 by Albert Einstein (for which he received Nobel Prize) based on Max Planck's hypothesis about the quantum nature of light. Einstein's work contained an important new hypothesis - if Planck suggested that light is emitted only in quantized portions, then Einstein already believed that light exists only in the form of quantum portions. From the concept of light as particles (photons), Einstein's formula for the photoelectric effect immediately follows:
where is the kinetic energy of the emitted electron, is the work function for given substance, is the frequency of the incident light, is Planck's constant, which turned out to be exactly the same as in Planck's formula for the radiation of an absolutely black body.
From this formula follows the existence of the red boundary of the photoelectric effect. Thus, studies of the photoelectric effect were among the earliest quantum mechanical studies.
Stoletov's laws
For the first time (1888–1890), analyzing in detail the phenomenon of the photoelectric effect, the Russian physicist A.G. Stoletov obtained fundamentally important results. Unlike previous researchers, he took a small potential difference between the electrodes. The scheme of Stoletov's experiment is shown in fig. 2.
Two electrodes (one in the form of a grid, the other flat), located in a vacuum, are attached to the battery. The ammeter included in the circuit is used to measure the resulting current strength. Irradiating the cathode with light of various wavelengths, Stoletov came to the conclusion that ultraviolet rays have the most effective effect. In addition, it was found that the strength of the current generated by the action of light is directly proportional to its intensity.
In 1898, Lenard and Thomson, using the method of charge deflection in electric and magnetic fields determined the specific charge of charged particles ejected 2. Scheme of Stoletov's experiment.
light from the cathode, and received the expression
SGSE unit s/g, coinciding with the known specific charge of the electron. From this it followed that under the action of light, electrons are ejected from the material of the cathode.
By summarizing the results obtained, the following patterns photoelectric effect:
1. With a constant spectral composition of light, the strength of the saturation photocurrent is directly proportional to the light flux incident on the cathode.
2. The initial kinetic energy of the electrons ejected by the light increases linearly with the frequency of the light and does not depend on its intensity.
3. The photoelectric effect does not occur if the frequency of light is less than a certain value characteristic of each metal, called the red border.
The first pattern of the photoelectric effect, as well as the occurrence of the photoelectric effect itself, can be easily explained based on the laws of classical physics. Indeed, the light field, acting on the electrons inside the metal, excites their oscillations. The amplitude of the forced oscillations can reach such a value at which the electrons leave the metal; then the photoelectric effect is observed.
In view of the fact that, according to the classical theory, the intensity of light is directly proportional to the square of the electric vector, the number of ejected electrons increases with increasing light intensity.
The second and third laws of the photoelectric effect are not explained by the laws of classical physics.
Studying the dependence of the photocurrent (Fig. 3), which occurs when a metal is irradiated with a stream of monochromatic light, on the potential difference between the electrodes (such a dependence is usually called the volt-ampere characteristic of the photocurrent), it was found that: 1) the photocurrent occurs not only at , but also at ; 2) the photocurrent is different from zero to a negative value of the potential difference strictly defined for a given metal, the so-called retarding potential; 3) the magnitude of the blocking (delaying) potential does not depend on the intensity of the incident light; 4) the photocurrent increases with decreasing absolute value of the retarding potential; 5) the value of the photocurrent increases with growth and from a certain value the photocurrent (the so-called saturation current) becomes constant; 6) the value of the saturation current increases with increasing intensity of the incident light; 7) the value of the delay 3. Feature
potential depends on the frequency of the incident light; photocurrent.
8) the speed of electrons ejected under the action of light does not depend on the intensity of light, but depends only on its frequency.
Einstein's equation
The phenomenon of the photoelectric effect and all its laws are well explained using the quantum theory of light, which confirms the quantum nature of light.
As already noted, Einstein (1905), developing quantum theory Planck put forward the idea that not only radiation and absorption, but also the propagation of light occurs in portions (quanta), the energy and momentum of which are:
where is the unit vector directed along the wave vector. Applying the law to the photoelectric effect in metals energy conservation, Einstein proposed the following formula:
, (1)
where is the work function of an electron from a metal, is the speed of a photoelectron. According to Einstein, each quantum is absorbed by only one electron, and part of the energy of the incident photon is spent on performing the work function of the metal electron, while the remaining part imparts kinetic energy to the electron.
As follows from (1), the photoelectric effect in metals can occur only at , otherwise the photon energy will be insufficient to eject an electron from the metal. The lowest frequency of light , under the influence of which the photoelectric effect occurs, is obviously determined from the condition
The light frequency determined by condition (2) is called the "red border" of the photoelectric effect. The word "red" has nothing to do with the color of light in which the photoelectric effect occurs. Depending on the type of metal, the “red border” of the photoelectric effect can correspond to red, yellow, violet, ultraviolet light, etc.
With the help of Einstein's formula, other regularities of the photoelectric effect can also be explained.
Let us assume that , i.e., there is a retarding potential between the anode and the cathode. If the kinetic energy of the electrons is sufficient, then they, having overcome the decelerating field, create a photocurrent. The photocurrent involves those electrons for which the condition is satisfied 
. The value of the retarding potential is determined from the condition
, (3)
where - maximum speed ejected electrons. Rice. 4.
Substituting (3) into (1), we obtain
Thus, the magnitude of the retarding potential does not depend on the intensity, but depends only on the frequency of the incident light.
The work function of electrons from a metal and Planck's constant can be determined by plotting the dependence on the frequency of the incident light (Fig. 4). As you can see, the segment cut off from the potential axis gives .
In view of the fact that the light intensity is directly proportional to the number of photons, an increase in the intensity of the incident light leads to an increase in the number of ejected electrons, i.e., to an increase in the photocurrent.
Einstein's formula for the photoelectric effect in non-metals has the form
.
The presence - the work of separation of a bound electron from an atom inside non-metals - is explained by the fact that, unlike metals, where there are free electrons, in non-metals, electrons are in a state bound to atoms. Obviously, when light falls on non-metals, part of the light energy is spent on the photoelectric effect in the atom - on the separation of the electron from the atom, and the rest is spent on the work function of the electron and imparting kinetic energy to the electron.
Conduction electrons do not spontaneously leave the metal in a noticeable amount. This is explained by the fact that the metal represents a potential well for them. It is possible to leave the metal only for those electrons whose energy is sufficient to overcome the potential barrier existing on the surface. The forces that cause this barrier have the following origin. The accidental removal of an electron from the outer layer of positive ions of the lattice leads to the appearance of an excess positive charge in the place that the electron left. The Coulomb interaction with this charge causes the electron, whose speed is not very high, to return back. Thus, individual electrons leave the metal surface all the time, move away from it by several interatomic distances, and then turn back. As a result, the metal is surrounded by a thin cloud of electrons. This cloud together with the outer layer of ions forms a double electric layer (Fig. 5; circles - ions, black dots - electrons). The forces acting on an electron in such a layer are directed inside the metal. The work done against these forces during the transfer of an electron from the metal to the outside goes to increase the potential energy of the electron (Fig. 5).
Thus, the potential energy of valence electrons inside the metal is less than outside the metal by an amount equal to the depth of the potential well (Fig. 6). The change in energy occurs over a length of the order of several interatomic distances; therefore, the walls of the well can be considered vertical.
Potential energy of an electron Fig. 6.
and the potential of the point where the electron is located have opposite signs. It follows from this that the potential inside the metal is greater than the potential in the immediate vicinity of its surface by .
Giving excess positive charge to the metal increases the potential both on the surface and inside the metal. The potential energy of an electron decreases accordingly (Fig. 7, a).
The values of the potential and potential energy at infinity are taken as the reference point. The introduction of a negative charge lowers the potential inside and outside the metal. Accordingly, the potential energy of the electron increases (Fig. 7, b).
The total energy of an electron in a metal is the sum of the potential and kinetic energies. At absolute zero, the values of the kinetic energy of conduction electrons range from zero to the energy coinciding with the Fermi level. On fig. 8, the energy levels of the conduction band are inscribed in the potential well (dotted lines show levels unoccupied at 0K). To move out of the metal, different electrons need to be given different energies. So, an electron located at the lowest level of the conduction band must be given energy; for an electron at the Fermi level, the energy is sufficient 
.
The smallest energy that must be imparted to an electron in order to remove it from a solid or liquid body into a vacuum is called exit work. The work function of an electron from a metal is determined by the expression
We have obtained this expression under the assumption that the temperature of the metal is 0K. At other temperatures, the work function is also defined as the difference between the depth of the potential well and the Fermi level, i.e., definition (4) is extended to any temperature. The same definition applies to semiconductors.
The Fermi level depends on temperature. In addition, due to the change in the average distances between atoms due to thermal expansion, the depth of the potential well slightly changes. This results in the work function being slightly temperature dependent.
The work function is very sensitive to the state of the metal surface, in particular to its purity. Having chosen properly Fig. eight.
surface coating, the work function can be greatly reduced. So, for example, deposition of an oxide layer of an alkaline earth metal (Ca, Sr, Ba) on the surface of tungsten reduces the work function from 4.5 eV (for pure W) to 1.5 - 2 eV.
Internal photoelectric effect
Above, we talked about the release of electrons from the illuminated surface of a substance and their transition to another medium, in particular, to vacuum. This emission of electrons is called photoelectronic emission, but the phenomenon itself external photoelectric effect. Along with it is also known and widely used for practical purposes, the so-called internal photoelectric effect, at which, unlike the external one, optically excited electrons remain inside the illuminated body without violating the neutrality of the latter. In this case, the concentration of charge carriers or their mobility changes in the substance, which leads to a change in the electrical properties of the substance under the action of light incident on it. The internal photoelectric effect is inherent only in semiconductors and dielectrics. It can be detected, in particular, by the change in the conductivity of homogeneous semiconductors when they are illuminated. Based on this phenomenon, photoconductivity created and constantly improved large group light receivers - photoresistors. They mainly use selenide and cadmium sulfide.
In inhomogeneous semiconductors, along with a change in conductivity, the formation of a potential difference is also observed (photo - emf). This phenomenon (photovoltaic effect) is due to the fact that, due to the homogeneity of the conductivity of semiconductors, there is a spatial separation inside the volume of the conductor of optically excited electrons that carry a negative charge and microzones (holes) that arise in the immediate vicinity of the atoms from which the electrons have been torn off, and like particles of carriers positive elemental charge. Electrons and holes are concentrated at different ends of the semiconductor, as a result of which electromotive force, thanks to which it is produced without the application of an external emf. electricity in a load connected in parallel with an illuminated semiconductor. In this way, a direct conversion of light energy into electrical energy is achieved. It is for this reason that photovoltaic light receivers are used not only for registering light signals, but also in electrical circuits as sources of electrical energy.
The main industrial types of such receivers operate on the basis of selenium and silver sulfide. Silicon, germanium and a number of compounds - GaAs, InSb, CdTe and others are also very common. Photovoltaic cells used for conversion solar energy into electricity, have become especially widely used in space research as sources of onboard power. They have relatively high coefficient useful action(up to 20%), very convenient in autonomous flight spaceship. In modern solar cells depending on the semiconductor material photo - emf. reaches 1 - 2 V, current removal from - several tens of milliamps, and for 1 kg of mass, the output power reaches hundreds of watts.
Demonstrates a simple experience. If a negatively charged zinc plate connected to an electroscope (a device that indicates the presence of electric charge), illuminated by the light of an ultraviolet lamp, then very quickly the needle of the electroscope will go to the zero state. This indicates that the charge has disappeared from the surface of the plate. If the same experiment is done with a positively charged plate, the electroscope needle will not deviate at all. This experiment was first carried out in 1888 by the Russian physicist Alexander Grigorievich Stoletov.
Alexander Grigorievich Stoletov
What happens to matter when light falls on it?
We know that light is electromagnetic radiation, a stream of quantum particles - photons. When electromagnetic radiation falls on a metal, part of it is reflected from the surface, and part is absorbed by the surface layer. When absorbed, a photon gives up its energy to an electron. Having received this energy, the electron does work and leaves the surface of the metal. Both the plate and the electron have a negative charge, so they repel each other and the electron flies off the surface.
If the plate is positively charged, the negative electron knocked out from the surface will be attracted by it again and will not leave its surface.
Discovery history
The photoelectric effect was discovered in early XIX century.
In 1839, the French scientist Alexandre Edmond Becquerel observed the photovoltaic effect at the interface between a metal electrode and a liquid (electrolyte).
Alexander Edmond Becquerel
In 1873, the English electrical engineer Smith Willoughby discovered that if selenium is exposed to electromagnetic radiation, its electrical conductivity changes.
Conducting experiments on the study of electromagnetic waves in 1887, the German physicist Heinrich Hertz noticed that a charged capacitor discharges much faster if its plates are illuminated with ultraviolet radiation.
Heinrich Hertz
In 1888, the German experimental physicist Wilhelm Galvaks discovered that when a metal is irradiated with short-wave ultraviolet radiation, the metal loses its negative charge, that is, the phenomenon of the photoelectric effect is observed.
A huge contribution to the study of the photoelectric effect was made by the Russian physicist Alexander Grigoryevich Stoletov, who conducted detailed experiments on the study of the photoelectric effect in 1888-1890. For this, he designed special device consisting of two parallel disks. One of these discs cathode, made of metal, was inside a glass case. another disk, anode, represented metal mesh applied to made of quartz glass body end. Quartz glass was chosen by scientists not by chance. The fact is that it transmits all types of light waves, including ultraviolet radiation. Ordinary glass delays ultraviolet radiation. Air was pumped out of the case. A voltage was applied to each of the disks: negative to the cathode, positive to the anode.
Stoletov's experience
During the experiments, the scientist illuminated the cathode through the glass with red, green, blue and ultraviolet light. The magnitude of the current was recorded by a galvanometer, in which the main element was a mirror. Depending on the magnitude of the photocurrent, the mirror was deflected by different angle. The greatest effect was exerted by ultraviolet rays. And the more of them there were in the spectrum, the stronger was the effect of light.
Stoletov discovered that only negative charges are released under the action of light.
The cathode was made from various metals. The most sensitive to light were such metals as aluminum, copper, zinc, silver, nickel.
In 1898, it was established that the negative charges released during the photoelectric effect are electrons.
And in 1905, Albert Einstein explained the phenomenon of the photoelectric effect as special case the law of conservation and transformation of energy.
external photoelectric effect
external photoelectric effect
The process of release of electrons from a substance under the action of electromagnetic radiation is called external photoelectric effect, or photoelectronic emission. Electrons emitted from the surface are called photoelectrons. Accordingly, the electric current that is generated during their ordered movement is called photocurrent.
The first law of the photoelectric effect
The strength of the photocurrent is directly proportional to the density of the light flux. The higher the radiation intensity, the large quantity electrons will be knocked out of the cathode in 1 s.
The intensity of the light flux is proportional to the number of photons. As the number of photons increases, the number of electrons that leave the metal surface and create a photocurrent increases. Therefore, the current increases.
The second law of the photoelectric effect
The maximum kinetic energy of electrons ejected by light increases linearly with the frequency of light and does not depend on its intensity.
The energy possessed by a photon incident on the surface is:
E = h ν ,where ν is the frequency of the incident photon; h is Planck's constant.
Getting energy E , the electron does work φ . The rest of the energy is the kinetic energy of the photoelectron.
From the law of conservation of energy follows the equality:
h ν=φ + W e , where We - the maximum kinetic energy of the electron at the moment of departure from the metal.
h ν=φ + m v2/2
The third law of the photoelectric effect
For each substance there is a red border of the photoelectric effect, that is, the minimum frequency of light νmin(or maximum wavelength λmax), at which the photoelectric effect is still possible, and if ν˂ νmin, then the photoelectric effect no longer occurs.
The photoelectric effect appears at a certain frequency of light. νmin . At this frequency, called "red" border of the photoelectric effect, the emission of electrons begins.
h ν min = φ .
If the frequency of the photon is lower νmin , its energy will not be enough to "knock out" an electron from the metal.
Internal photoelectric effect
If, under the influence of radiation, electrons lose contact with their atoms, but do not leave solid and liquid semiconductors and dielectrics, but remain inside them as free electrons, then such a photoelectric effect is called internal. As a result, the electrons are redistributed along energy states. The concentration of charge carriers changes and arises photoconductivity(increase in conductivity under the influence of light).
The internal photoelectric effect is also referred to as valve photoelectric effect, or photoelectric effect in the barrier layer. This photoelectric effect occurs when, under the influence of light, electrons leave the surface of the body and pass into another, contacting body - a semiconductor or electrolyte.
Applying the Photo Effect
All devices based on the photoelectric effect are called photocells. The world's first photocell was Stoletov's device, which he created to conduct experiments on the study of the photoelectric effect.
Photovoltaic cells are widely used in the most various devices in automation and telemechanics. Without photocells, it is impossible to control machine tools with numerical control (CNC), which can create parts according to drawings without human intervention. With their help, sound is read from the film. They are part of various control devices, help to stop and block the device at the right time. With photocells street lighting turns on at dusk and turns off at dawn. They help control the turnstiles in the metro and beacons on land, lower the barrier when the train approaches the crossing. They are used in telescopes and solar panels.
Theory
The photoelectric effect is the ejection of electrons from a substance by the action of light. In a metal, an electron moves freely, but when it leaves the surface, the metal itself is charged with a positive charge because of this and prevents it from escaping. Therefore, in order to leave the metal, the electron must have additional energy, depending on the substance. This energy is called the work function.
To study the photoelectric effect, you can assemble the setup shown in Fig. 1. It consists of a glass container from which the air is pumped out. The window through which the light falls is made of quartz glass, which transmits visible and ultraviolet rays. Two electrodes are soldered inside the balloon: one of which - the cathode - is illuminated through the window. Between the electrodes, the source creates an electric field that causes photoelectrons to move from the cathode to the anode.
moving electrons form an electric current (photocurrent). When the voltage changes, the current changes. dependency graph I from U- current-voltage characteristic - shown in fig. 2. At low voltages, not all electrons torn out of the cathode reach the anode; as the voltage increases, their number increases. 
At a certain voltage, all the electrons torn out by light reach the anode, then the saturation current is set I n, with a further increase in voltage, the current does not change.
With an increase in the intensity of the incident radiation, an increase in the saturation current is observed, which is proportional to the number of ejected electrons. The 1st law of the photoelectric effect states that the number of electrons ejected by light from the surface of a metal is proportional to the absorbed energy of the light wave.
To measure the kinetic energy of electrons, you need to change the polarity of the current source. On the graph, this case corresponds to the section at U , at which the photocurrent drops to zero. Now the field does not accelerate, but slows down the photoelectrons. At some voltage, called retarding U 3, the photocurrent disappears. In this case, all the electrons will be stopped by the field, then the field will return them to the former cathode, just as a stone thrown upwards will be stopped by the Earth's gravitational field and returned back to the Earth.
The work of the forces of the electric field A = qU 3, spent on deceleration of the electron, is equal to the change in the kinetic energy of the electron, that is m v 2 /2 = qU 3, where m- electron mass, v - its speed, q- charge. That is, by measuring the retarding voltage U 3, we determine the maximum kinetic energy. It turned out that the maximum kinetic energy of electrons does not depend on the intensity of light, but only on frequency. This statement is called the 2nd law of the photoelectric effect.
At a certain limiting frequency of light, which depends on a particular substance, and at lower frequencies, the photoelectric effect is not observed. This boundary frequency is called the "red" boundary of the photoelectric effect.
A. Einstein explained the laws of the photoelectric effect in 1905. He used Planck's idea of the quantum nature of light. The energy of one quantum of light E = hν. If we assume that one quantum of light pulls out one electron, then the energy of the quantum E goes to do the work of the electron A and to give him kinetic energy mv 2 /2. That is
hν = A + mv 2 /2.
This equation is called the Einstein equation for the photoelectric effect.
Let us explain from the standpoint of Einstein's idea the 1st law of the photoelectric effect. If one quantum of energy pulls out one electron, then the more quantums the substance absorbs (the greater the intensity of light), the more electrons will fly out of the substance.
Explain the second law of the photoelectric effect. Work function A depends on the type of substance and does not depend on the frequency of light. The kinetic energy of an electron pulled out of a substance is mv 2 /2=h - A depends on the frequency of the light ν : the higher the frequency, the more kinetic energy the electron will receive. The intensity of light does not affect the kinetic energy of an electron, because the Einstein equation describes the energy of a single electron. It doesn't matter how many electrons fly out, the speed of each of them depends on the frequency.
Einstein's formula also explains the fact that light of a given frequency can pull out an electron from one substance, but not from another. For each substance, the photoelectric effect is observed if the energy of a light quantum is greater than or, in extreme cases, equal to the work function ( hν ≥ A). The limiting frequency at which the photoelectric effect is still possible, ν min = A/h. This is the frequency at which electrons are ejected without imparting kinetic energy to them - the frequency of the "red border" of the photoelectric effect.
We write the Einstein equation for the case when the kinetic energy of an electron is equal in magnitude to the work of the electric field forces, that is, at a retarding voltage:
hν = A + qU 3.
From here U 3 \u003d -A / q + (h / q) ν.
Let's build a graph of the dependence of the retarding voltage on the frequency (Fig. 3). It can be seen from the formula that the dependence U 3 from ν is linear. The tangent of the slope of the graph:
tan α \u003d ΔU 3 / Δν \u003d h / q.
Hence Planck's constant:
h = qtg α = q ΔU 3 /Δν.
This formula is for experimental definition Planck's constant.
The photoelectric effect is the phenomenon of pulling out the light of electrons from the metal (external)
A photoelectric effect is the emission of electrons by a substance under the action of light (or any other electromagnetic radiation). In condensed substances (solid and liquid), external and internal photoelectric effects are distinguished.
The external photoelectric effect (photoelectron emission) is the emission of electrons by a substance under the action of electromagnetic radiation. Electrons emitted from a substance during an external photoelectric effect are called photoelectrons, and the electric current generated by them during ordered motion in an external electric field is called the photocurrent.
The internal photoelectric effect is the redistribution of electrons over energy states in solid and liquid semiconductors and dielectrics, which occurs under the action of radiation. It manifests itself in a change in the concentration of charge carriers in the medium and leads to the appearance of photoconductivity or the valve photoelectric effect.
Photoconductivity is an increase in the electrical conductivity of a substance under the action of radiation.
The valve photoelectric effect is a kind of internal photoelectric effect - this is the occurrence of EMF (photo EMF) when illuminating the contact of two different semiconductors or a semiconductor and a metal (in the absence of an external electric field). The valve photoelectric effect opens the way for the direct conversion of solar energy into electrical energy.
The multiphoton photoelectric effect is possible if the light intensity is very high (for example, when using laser beams). In this case, an electron emitted by a metal can simultaneously receive energy not from one, but from several photons.
Stoletov's laws
First law
Investigating the dependence of the current strength in the balloon on the voltage between the electrodes at a constant luminous flux on one of them, he established the first law of the photoelectric effect.
The saturation photocurrent is proportional to the light flux incident on the metal.
Because the current strength is determined by the magnitude of the charge, and the luminous flux is determined by the energy of the light beam, then we can say:
the number of electrons knocked out of a substance in 1 s is proportional to the intensity of the light falling on this substance.
Second law
Changing the lighting conditions at the same installation, A. G. Stoletov discovered the second law of the photoelectric effect: the kinetic energy of photoelectrons does not depend on the intensity of the incident light, but depends on its frequency.
It followed from the experiment that if the frequency of light is increased, then with a constant light flux, the blocking voltage increases, and, consequently, the kinetic energy of photoelectrons also increases. Thus, the kinetic energy of photoelectrons increases linearly with the frequency of light.
third law
Replacing the material of the photocathode in the device, Stoletov established the third law of the photoelectric effect: for each substance there is a red border of the photoelectric effect, i.e. there is a minimum frequency nmin at which the photoelectric effect is still possible.
The law of conservation of energy, written down by Einstein for the photoelectric effect, is the statement that the energy of a photon, acquired by an electron, allows it to leave the surface of the conductor, having done the work function. The rest of the energy is realized in the form of the kinetic energy of the now free electron
The energy of the incident photon is expended on the electron performing the work function A from the metal and on communicating the kinetic energy mv2max/2 to the emitted photoelectron. According to the law of conservation of energy,
(203.1)
Equation (203.1) is called the Einstein equation for the external photoelectric effect.
Compton effect
Change in the wavelength of light upon scattering by bound electrons
EXPERIMENTS OF RUTERFORD. PLANETARY MODEL OF THE ATOM
Rutherford's experiments. The mass of electrons is several thousand times less than the mass of atoms. Since the atom as a whole is neutral, therefore, the bulk of the atom falls on its positively charged part.
In 1906 Ernest Rutherford proposed to use the probing of the atom with the help of -particles to experimentally study the distribution of the positive charge, and hence the mass inside the atom. These particles arise from the decay of radium and some other elements. Their mass is about 8000 times the mass of the electron, and the positive charge is equal in modulus to twice the charge of the electron. These are nothing but fully ionized helium atoms. The speed of -particles is very high: it is 1/15 of the speed of light.
With these particles, Rutherford bombarded the atoms of heavy elements. Electrons, due to their small mass, cannot noticeably change the trajectory of the -particle, just as a pebble of several tens of grams in a collision with a car cannot significantly change its speed.
Planetary model of the atom. Based on his experiences, Rutherford created planetary model atom. In the center of the atom is located a positively charged nucleus, in which almost all the mass of the atom is concentrated. In general, the atom is neutral. Therefore, the number of intraatomic electrons, as well as the charge of the nucleus, is equal to the ordinal number of the element in periodic system. It is clear that the electrons cannot rest inside the atom, since they would fall on the nucleus. They move around the core, just as the planets revolve around the sun. This character of the electron motion is determined by the action of the Coulomb forces of attraction from the side of the nucleus.
5. . 6. .
In 1900, the German physicist Max Planck hypothesized that light is emitted and absorbed in separate portions - quanta(or photons). The energy of each photon is determined by the formula , where is Planck's constant equal to , is the frequency of light. Planck's hypothesis explained many phenomena: in particular, the phenomenon of the photoelectric effect, discovered in 1887 by the German scientist Heinrich Hertz and experimentally studied by the Russian scientist Alexander Grigoryevich Stoletov.
photoelectric effect- This is the phenomenon of the emission of electrons by a substance under the influence of light. If you charge a zinc plate attached to an electrometer negatively and illuminate it with an electric blow (Fig. 35), then the electrometer will quickly discharge.
As a result of the research, the following empirical patterns were established:
The number of electrons ejected by light from the metal surface in 1 s is directly proportional to the energy of the light wave absorbed during this time;
The maximum kinetic energy of photoelectrons increases linearly with the frequency of light and does not depend on its intensity.
In addition, two fundamental properties were established.
Firstly, the inertia of the photoelectric effect: the process begins immediately at the moment the illumination begins.
Secondly, the presence of a minimum frequency characteristic of each metal - red border photo effect. This frequency is such that at , the photoelectric effect does not occur at any light energy, and if , then the photoelectric effect begins even at low energy.
The theory of the photoelectric effect was created by the German scientist A. Einstein in 1905. Einstein's theory is based on the concept of the work function of electrons from a metal and the concept of quantum light emission. According to Einstein's theory, the photoelectric effect has the following explanation: by absorbing a quantum of light, an electron acquires energies. When leaving the metal, the energy of each electron decreases by a certain amount, which is called work function(). The work function is the work required to remove an electron from a metal. Therefore, the maximum kinetic energy of electrons after departure (if there are no other losses) is equal to: . Hence,
.
This equation is called Einstein's equations.
Devices based on the principle of operation of which is the phenomenon of the photoelectric effect are called photocells. The simplest such device is a vacuum photocell. The disadvantages of such a photocell are low current, low sensitivity to long-wave radiation, difficulty in manufacturing, impossibility of use in circuits. alternating current. It is used in photometry to measure the intensity of light, brightness, illumination, in cinema to reproduce sound, in phototelegraphs and phototelephones, in the management of production processes.
There are semiconductor photocells, and in which, under the influence of light, the concentration of current carriers changes. They are used in automatic control electrical circuits(for example, in subway turnstiles), in alternating current circuits, as non-renewable current sources in watches, microcalculators, the first solar cars are being tested, used in solar batteries on artificial Earth satellites, interplanetary and orbital automatic stations.
The phenomenon of the photoelectric effect is associated with photochemical processes occurring under the action of light in photographic materials.