The photoelectric effect easily explained with 2 examples

The photoelectric effect

At the end of the last century, a scientist discovered the phenomenon, electron emission from a metal surface when it is illuminated by visible or ultraviolet. The photoelectric effect is the appearance of an electron (photoelectron) erupting from metal by light. To be more precise, the photoelectric effect (photo effect) is the phenomenon that electrons leave a metal when it is illuminated by visible or ultraviolet light. This phenomenon was later found on other materials as well.

Einstein was the first to explain the phenomenon. The photoelectric effect occurs when photons, or as particles are popularly called, collide with electrons in metal atoms, giving them all their energy. The photon energy is spent on the output and the kinetic energy of the photoelectron.

The image above shows the apparatus for studying the photoelectric effect. In an airless glass balloon, two electrodes were heated, which were connected to a direct voltage. The voltage can be varied through a potentiometer and measured by a voltmeter. A galvanometer is a measuring electric current. When the cathode (electrode connected to the negative pole of the battery) is illuminated, for example, by ultraviolet light, electrons are emitted from its surface under the action of an electric field, they move towards the anode, and an electric current (photocurrent) flows through the circuit.

Current-Voltage graphic

The graph below shows the dependence of the photocurrent intensity on the voltage at the electrodes. For two different intensities of incident monochromatic light on the same metal (the upper curve corresponds to a higher intensity of incident light). From the curves on the graph, we can make three conclusions.

First, the photocurrent strength increases with increasing electrical voltage, and at a saturation voltage reaches a constant value (set intensity of incident light), which does not increase with further voltage increase. It corresponds to the case when all the emitted electrons are accumulated around the anode.

Second, the magnitude of the amplification current is proportional to the intensity of the incident light.

Third, some photocurrent still flows at zero electric voltage since emitted photoelectrons have sufficient kinetic energy to reach the anode. This opposite voltage gradually increases, and the photocurrent decreases. To prevent these electrons from reaching the anode we tied them to the negative pole of the battery. At a voltage braking, all photoelectrons will be retained on the emitting electrode. Then, the photocurrent will stop flowing. In this case, the work that performs the braking field is equal to the maximum kinetic energy of the emitted photoelectrons (Ek) max.

The photoelectric effect
Photoelectric effect- Graph

The shortest time

The smallest or the shortest fraction of time, compared to which the blink of an eye looks like an eternity, was measured recently based on electrons fleeing from atoms. Each blink lasts from 100 to 150 milliseconds or thousandths of a second. This also shows the quantum nature of light.

Scientists have measured changes in the atom at the level of zeptoseconds (one sextillion seconds), which is the trillionth of a billionth of a second – the smallest period observed so far. In this case, the speed devil was an electron that broke the bond with its parent atom. When a light beam hits electrons, they become excited and break ties with atoms. This electronic ejection is known as the photoelectric effect.

This interpretation attributes corpuscular (particle) properties to light, although this is a quasi-particle, a particle without mass, which exists when moving. In other words, electromagnetic radiation, depending on the phenomenon, manifests a dualistic (both wave and particle) character. Photoelectrons have enough energy to go outside. Under the action of photons, electrons erupt from the depleted region of the PN compound (usually silicon) and travel according to the negative busbar, and the positive particles (cavities) positive busbar- EM force


An experiment shows the maximum kinetic energy of an electron doesn’t depend on the intensification of the light. However, it depends only that its frequencies and the boiling point of the material of the cashew. A classical electromagnetic theory can’t explain the photoelectric effect. According to this theory, the electromagnetic radiation that falls on a material excites electrons to a forced oscillation. The oscillation energy of the electron is proportional to the square of the amplitude of the incident radiation. At the sufficiently high intensity of the incident light, the bonds of electrons from the substance would be broken. (The energy required to break that bond and expel the electron from the substance is called output work.)

According to this understanding, the light of arbitrary frequency, but sufficient intensity, causes a photoelectric effect. The maximum kinetic would also be the ejected electrons energy that should increase with a further intensity increase. However, the experimental facts were different. Applying the idea of ​​photons, Einstein gave a complete explanation of the photo effect in 1905. According to that theory, the photoelectric effect is caused by the collision of a photon with an electron on the surface of the material, whereby the photon transfers all its energy to one electron. The application of energy is spent partly on the work of separating the electron from the substance, and the rest is converted into the kinetic energy of the photoelectron.


In the case of alkali metals, the threshold of the photoelectric effect falls in the visible region of the spectrum, and the case of most other metals – in the ultraviolet region. Today, materials with a photoelectric effect threshold in the infrared part of the spectrum are known. For example, cellulose compounds with antimony or bismuth, semiconductor layers, and many others.

The number of emitted photoelectrons, and thus the strength of the photocurrent, depends on the number of incident photons per unit of time. And that only means that it also depends on the intensity of light.

Today, the photo effect is widely used in the technique for the detection and measurement of optical radiation, which is based on the conversion of light intensities into electrical ones. For example, the photoelectric effect is based on the operation of photocells, photoaccumulators, image intensifiers, and many other devices.

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