give the statement for law of Photoelectric effect in the basic of experimental observation
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In the photoelectric effect, electrons are emitted from solids, liquids or gases when they absorb energy from light. Electrons emitted in this manner may be called photoelectrons.
In 1887, Heinrich Hertz discovered that electrodes illuminated with ultraviolet light create electric sparks more easily. In 1905 Albert Einstein published a paper that explained experimental data from the photoelectric effect as being the result of light energy being carried in discrete quantized packets. This discovery led to the quantum revolution (and a Nobel Prize for Einstein in 1921). The photoelectric effect depends on the "particle picture" of light in that it seems to confirm Max Planck's previous discovery of the Planck relation linking energy (E) and frequency and Planck constant. These quanta, or discrete values of energy associated with light waves have energy equal to
{\displaystyle E=hf} {\displaystyle E=hf}
where {\displaystyle f} f is the frequency (cycles per second) of the light striking the metal plate, and {\displaystyle h} h is Planck’s constant.
Experimental observations of photoelectric emission
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For a given metal, there exists a certain minimum frequency of incident radiation below which no photoelectrons are emitted. This frequency is called the threshold frequency. Increasing the frequency of the incident beam, keeping the number of incident photons fixed (this would result in a proportionate increase in energy) increases the maximum kinetic energy of the photoelectrons emitted. Thus the stopping voltage increases. If the intensity of the incident radiation is increased, there is no effect on the kinetic energies of the photoelectrons. The kinetic energy depends only on the frequency on the light, and hence the energy provided by each photon. This suggests that the photoelectric effect involves one electron and only one photon.
Above the threshold frequency, the maximum kinetic energy energy of the emitted photoelectron depends on the frequency of the incident light, but is independent of the intensity of the incident light so long as the latter is not too high. On the other hand, the stopping voltage does not depend on the intensity of the light (provided the light's frequency is kept fixed.)Here we show how the work function of the metal, as well as the numerical value of Plank's constant can be experimentally deduced from a plot of stopping voltage versus photon frequency. The maximum kinetic energy {\displaystyle K_{\mathrm {max} }} {\displaystyle K_{\mathrm {max} }} equals the energy that the electron received from the photon minus the energy required to remove the electron from the metal.
{\displaystyle K_{\mathrm {max} }=h\,f-\varphi ,} {\displaystyle K_{\mathrm {max} }=h\,f-\varphi ,}
where {\displaystyle h} h is the Planck constant and {\displaystyle f} f is the frequency of the incident photon. The term {\displaystyle \varphi } {\displaystyle \varphi } is the work function, which gives the minimum energy required to remove a delocalised electron from the surface of the metal.
The important feature of this equation is that it resembles the formula for a straight line ( {\displaystyle y=mx+b} {\displaystyle y=mx+b}). The maximum kinetic energy can be calculated using the voltage required to prevent the electrons from passing between the two electrodes. This minimum voltage to stop all electrons is called the stopping voltage. Plank's constant can be calculated as the slope when stopping voltage is plotted as a function of frequency.
It is interesting how a straight line of a graph of stopping voltage versus frequency is used to measure one of the most mysterious fundamental constants in nature.