For any particular material, there is a threshold frequency that must be exceeded, independent of light intensity, to observe any electron emission. Three-step model[ edit ] In the X-ray regime, the photoelectric effect in crystalline material is often decomposed into three steps: [10] —51 Inner photoelectric effect see photo diode below[ clarification needed ]. The hole left behind can give rise to the Auger effect , which is visible even when the electron does not leave the material. In molecular solids phonons are excited in this step and may be visible as lines in the final electron energy.

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Incoming EM radiation on the left ejects electrons, depicted as flying off to the right, from a substance. Upon exposing a metallic surface to, and absorption of, electromagnetic radiation that is above the threshold frequency particular to each type of surface a current is produced.

The electrons that are emitted are often termed photoelectrons in many textbooks. The photoelectric effect helped further wave-particle duality , whereby physical systems such as photons in this case can display both wave-like and particle-like properties and behaviours, a concept that was used by the creators of quantum mechanics. The photoelectric effect was explained mathematically by Albert Einstein extending the work on quanta developed by Max Planck. History Early observations In , Alexandre Edmond Becquerel observed the photoelectric effect via an electrode in a conductive solution exposed to light.

In , Willoughby Smith found that selenium is photoconductive. His receiver consisted of a coil with a spark gap , whereupon a spark would be seen upon detection of EM waves. He placed the apparatus in a darkened box in order to see the spark better; he observed, however, that the maximum spark length was reduced when in the box. A glass panel placed between the source of EM waves and the receiver absorbed ultraviolet radiation that assisted the electrons in jumping across the gap.

When removed, the spark length would increase. He observed no decrease in spark length when he substituted quartz for glass, as quartz does not absorb UV radiation. Hertz concluded his months of investigation and reported the results obtained. He did not further pursue investigation of this effect, nor did he make any attempt at explaining how the observed phenomenon was brought about. Influenced by the work of James Clerk Maxwell , Thomson deduced that cathode rays consisted of negatively charged particles, which he called "corpuscles" later called "electrons".

In the research, Thomson enclosed a metal plate i. The current and speed of this current varied with the intensity and color of the radiation. Larger increments of the radiation intensity or frequency of the field would produce more current. He used a powerful electric arc lamp which enabled him to investigate large changes in intensity, and had sufficient power to enable him to investigate the variation of potential with light frequency.

His experiment directly measured potentials, not electron kinetic energy: he found the electron energy by relating it to the maximum stopping potential voltage in a phototube. He found that the calculated maximum electron kinetic energy is determined by the frequency of the light. For example, an increase in frequency results in an increase in the maximum kinetic energy calculated for an electron upon liberation - ultraviolet radiation would require a higher applied stopping potential, to stop current in a phototube, than blue light.

Doubling the intensity of the light doubled the number of electrons emitted from the surface. Lenard did not know of photons. This paper proposed the simple description of "light quanta" later called "photons" and showed how they could be used to explain such phenomena as the photoelectric effect. The simple explanation by Einstein in terms of absorption of single quanta of light explained the features of the phenomenon and helped explain the characteristic frequency.

Einstein, by assuming that light actually consisted of discrete energy packets, wrote an equation for the photoelectric effect that fit experiments.

This was an enormous theoretical leap and the reality of the light quanta was strongly resisted. Perhaps surprisingly, that had not yet been tested. Effect on wave-particle question The photoelectric effect helped propel the then-emerging concept of the dual nature of light light exhibits characteristics of waves and particles at different times.

It was impossible to understand in terms of the classical wave description of light, as the energy of the emitted electrons did not depend on the intensity of the incident radiation. For such a classical theory to work a pre-loaded state would need to persist in matter. These ideas were abandoned. Explanation The photons of the light beam have a characteristic energy given by the wavelength of the light.

In the photoemission process, if an electron absorbs the energy of one photon and has more energy than the work function , it is ejected from the material. If the photon energy is too low, however, the electron is unable to escape the surface of the material. Increasing the intensity of the light beam does not change the energy of the constituent photons, only their number, and thus the energy of the emitted electrons does not depend on the intensity of the incoming light.

Electrons can absorb energy from photons when irradiated, but they follow an "all or nothing" principle. All of the energy from one photon must be absorbed and used to liberate one electron from atomic binding, or the energy is re-emitted.

When this equation is not observed to be true that is, the electron is not emitted or it has less than the expected kinetic energy , it may be because when given an excess amount of energy to the body, some energy is absorbed as heat or emitted as radiation, as no system is perfectly efficient. Uses and effects Solar cells used in solar power and light-sensitive diodes use the photoelectric effect.

They absorb photons from light and put the energy into electrons, creating electric current. Electroscopes Electroscopes are fork-shaped, hinged metallic leaves placed in a vacuum jar, partially exposed to the outside environment. When an electroscope is charged positively or negatively, the two leaves separate, as charge distributes evenly along the leaves causing repulsion between two like poles. When ultraviolet radiation or any radiation above threshold frequency is shone onto the metallic outside of the electroscope, the negatively charged one will discharge and collapse, while nothing will happen to the positively charged one.

The reason is that electrons will be liberated from the negatively charged one, gradually making it neutral, while liberating electrons from the positively charged one will make it even more positive, keeping the leaves apart.

This must be done in a high vacuum environment, since the electrons would be scattered by air. A typical electron energy analyzer is a concentric hemispherical analyser CHA , which uses an electric field to divert electrons different amounts depending on their kinetic energies.

For every element and core atomic orbital there will be a different binding energy. The many electrons created from each will then show up as spikes in the analyzer, and can be used to determine the elemental composition of the sample. Template:Ref Spacecraft The photoelectric effect will cause spacecraft exposed to sunlight to develop a positive charge. This can get up to the tens of volts. This can be a major problem, as other parts of the spacecraft in shadow develop a negative charge up to several kilovolts from nearby plasma, and the imbalance can discharge through delicate electrical components.

The static charge created by the photoelectric effect is self-limiting, though, because a more highly-charged object gives up its electrons less easily. Template:Ref Moon dust Light from the sun hitting lunar dust causes it to become charged through the photoelectric effect.

The charged dust then repels itself and lifts off the surface of the moon by electrostatic levitation. This manifests itself almost like an "atmosphere of dust", visible as a thin haze and blurring of distant features, and visible as a dim glow after the sun has set. This was first photographed by the lunar surveyor in the s.

It is thought that the smallest particles are repelled up to kilometers high, and that the particles move in "fountains" as they charge and discharge. Template:Ref Template:Ref See also.


Effet Compton

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Effet photoélectrique



Effet Photoelectrique



Effet Compton


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