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The photoelectric effect Compton Effect

comes from: Quantum Physics: Waves are particles

In 1905 Albert Einstein explained the photoelectric effect, independently deducting Planck radiation consisted of particles with an energy proportional to its frequency, and contributing to the development of quantum physics. For this development, Einstein received Nobel Prize in 1928.

In the experiment, a beam of light strikes a metal. The action of radiation is electrons start of the plate, that the application of an electric field, leading to another plate "collector", to record an electric current in an ammeter, a way of quantifying the electrons that are emitted

What was found experimentally was:

- The electrons are emitted when the frequency of the light reaches a minimum value. Below this threshold, not output, regardless of the intensity of light.
- by applying a voltage of "braking" V between the collector and emitter, electrons can be slowed by this potential energy. Increasing this voltage, the intensity decreases, reaching a maximum value above which no electrons reach the collector, and the intensity is zero. This maximum value depends on the frequency, and is greater, the higher the frequency of radiation.

The explanation is simple if you consider light as a particle, a photon. Has a particular energy proportional to its frequency. This is absorbed by the electron, which must:
a) First out of the material. This costs an energy, called the work function.
b) Second, to move. With energy after spending the surplus needed to leave the material, the electron is set in motion with a kinetic energy.

hν = Φ + E _{c
c E = hν-Φ}

Given a material with a work function Φ a photon should at least provide the energy to start the electron. If the photon energy is less than the work function, the electron can not be torn from the material. If the energy is greater than this value, then the excess energy is transformed into energy the electron kinetic moves. In the limiting case where all the energy is used to overcome the work function, but no kinetic energy to move:

The minimum frequency to produce the photoelectric effect is given by Φ / h

applying the brake electric potential V, the electron loses energy on its way, so that only those who will traverse the shortest path between the emitter and collector plate. By increasing the retarding potential, then you can get to the point that even these latter are able to reach. Then eliminating the potential energy of the electron in an amount equal to the kinetic energy that left the electron.


In the limiting case where the voltage stops all electrons, the kinetic energy equals the potential energy created by braking voltage, eV, where e is the electron charge.

_{c eV = E = hν-Φ}

In this way the observed phenomena are explained the photoelectric effect.