Einstein equation

1-st law of a photoeffect: quantity of the electrons which are pulled out by light from a surface of metal for 1с, in direct proportion intensity of light.

To according to 2-nd law of a photoeffect, the maximal kinetic energy of electrons pulled out by light linearly will increase with frequency of light and does not depend on its intensity.

3-rd law of a photo effect: for each substance there is a red border of a photo effect. The minimal frequency of light v₀ (or the maximal length of a wave λ₀, аt which the photo effect, and if is still possible v < v₀, that the photo effect does not occur any more.

The first law is explained from a position of the electromagnetic theory of light: the more intensity of a light wave, a lot of electrons will be passed sufficient for a start from metal energy. Other laws of a photo effect contradict this theory.

The theoretical explanation of these laws has been given in 1905 Einstein.

According to it, electromagnetic radiation represents a stream of separate quantums (photons) with energy hv (h - Planck constant.)

At a photo effect the part of falling electromagnetic radiation from a surface of metal is reflected, and the part gets inside of a superficial layer of metal and there is absorbed. Having absorbed a photon, electron receives from it energy and, making work of an output, leaves metal:

hv=A+mv² / 2

mv² - the maximal kinetic energy which can have electron at a start from metal. It is maybe certain:

mv²/2 = eUd

Ud - detaining voltage

In Einstein's theory laws of a photo effect speak as follows:

Intensity of light is proportional to number of photons in a light bunch and consequently defines number of the electrons which have been pulled out from metal.

The second law follows from the equation

mv² /2=hv-A

From the same equation follows, that the photo effect is possible only in that case when energy of the absorbed photon exceeds work of an output of electron on metal. е. Frequency of light thus should exceed some value certain for everyone substance.

This minimal frequency defines red border of a photoeffect. At smaller frequency of light of energy of a photon does not suffice for fulfilment by electron of work of an output and consequently the photo effect is absent. Einstein's quantum theory has allowed to explain and one more law established by Stoletov. In 1888 Stoletov has noticed, that the photocurrent appears almost at the same time with illumination of the cathode of a photo cell.

Under the classical wave theory in a floor of a light electromagnetic wave time for accumulation of energy necessary for a start and consequently the photo effect should proceed with delay on an extreme measure for some seconds is required to electron.

Under the quantum theory when the photon is absorbed by electron all energy of a photon passes to electron and any time for accumulation of energy it is not required.

With the invention of lasers there was a possibility to experiment very intensive bunches of light. Applying supershort impulses of laser radiation, it was possible to observe multiphoton processes when electron, before to leave the cathode, underwent impact not with one, and with several photons.

In this case the equation of a photo effect is recorded

Nhv=A+mv² /2.

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