Photoelectric Effect
This effect can be attributed to the transfer of energy from the light to an electron in the metal. From this perspective, an alteration in either the intensity or wavelength of light would induce changes in the rate of emission of electrons from the metal.
Furthermore, according to this theory, a sufficiently dim light would be expected to show a time lag between the initial shining of its light and the subsequent emission of an electron. However, the experimental results did not correlate with either of the two predictions made by classical theory.
The electron emission from the surface of a metal is possible only if sufficient addition energy (equal to work function of the sources such as heat energy, energy stored in electric field, light energy or kinetic energy of the electric charges bombarding the metal surface. Accordingly; there are following four principal method of obtaining electron emission from the surface of a metal:
Electrons are only dislodged by the impingement of photons when those photons reach or exceed a threshold frequency. Below that threshold, no electrons are emitted from the metal regardless of the light intensity or the length of time of exposure to the light.
To make sense of the fact that light can eject electrons even if its intensity is low, Albert Einstein proposed that a beam of light is not a wave propagating through space, but rather a collection of discrete wave packets (photons), each with energy hf.
This shed light on Max Planck's previous discovery of the Planck relation (E = hf) linking energy (E) and frequency (f) as arising from quantization of energy. The factor h is known as the Planck constant.
Einstein's Equations for the Photoelectric Effect
Einstein's interpretation of the photoelectric effect results in equations which are valid for visible and ultraviolet light:
Energy of photon = energy needed to remove an electron + kinetic energy of the emitted electron
hν = W + E
where
h is Planck's constant
ν is the frequency of the incident photon
W is the work function, which is the minimum energy required to remove an electron from the surface of a given metal: hν0
E is the maximum kinetic energy of ejected electrons: 1/2 mv2
ν0 is the threshold frequency for the photoelectric effect
m is the rest mass of the ejected electron
v is the speed of the ejected electron
No electron will be emitted if the incident photon's energy is less than the work function.
Applying Einstein's special theory of relativity, the relation between energy (E) and momentum (p) of a particle is E = [(pc)2 + (mc2)2](1/2) where m is the rest mass of the particle and c is the velocity of light in a vacuum.
History of Photoelectric Effect
photoelectric effect is the emission of electrons or other free carriers when light is shone onto a material. Electrons emitted in this manner can be called photo electrons. The phenomenon is commonly studied in electronic physics, as well as in fields of chemistry, such as quantum chemistry orelectrochemistry.According to classical electromagnetic theory, this effect can be attributed to the transfer of energy from the light to an electron.
From this perspective, an alteration in theintensity of light would induce changes in the kinetic energy of the electrons emitted from the metal. Furthermore, according to this theory, a sufficiently dim light would be expected to show a time lag between the initial shining of its light and the subsequent emission of an electron. However, the experimental results did not correlate with either of the two predictions made by classical theory.Instead, electrons are dislodged only by the impingement of photons when those photons reach or exceed a thresholdfrequency (energy).
Below that threshold, no electrons are emitted from the metal regardless of the light intensity or the length of time of exposure to the light (rarely, an electron will escape by absorbing two or more quanta. However, this is extremely rare because by the time it absorbs enough quanta to escape, the electron will probably have emitted the rest of the quanta.).
To make sense of the fact that light can eject electrons even if its intensity is low, Albert Einstein proposed that a beam of light is not a wave propagating through space, but rather a collection of discrete wave packets (photons), each with energy hν. This shed light on Max Planck's previous discovery of the Planck relation(E = hν) linking energy (E) and frequency (ν) as arising from quantization of energy. The factor h is known as the Planck constant.
In 1887, Heinrich Hertz
Discovered that electrodesilluminated with ultraviolet light create electric sparks more easily. In 1900, while studyingblack-body radiation, the German physicist Max Plancksuggested that the energy carried by electromagnetic waves could only be released in "packets" of energy.
In 1905,Albert Einstein published a paper advancing the hypothesis that light energy is carried in discrete quantized packets to explain experimental data from the photoelectric effect. This model contributed to the development of quantum mechanics. In 1914, Robert Millikan's experiment supported Einstein's model of the photoelectric effect.
Einstein was awarded the Nobel Prize in 1921 for "his discovery of the law of the photoelectric effect" and Millikan was awarded the Nobel Prize in 1923 for "his work on the elementary charge of electricity and on the photoelectric effect".The photoelectric effect requires photons with energies approaching zero (in the case of negative electron affinity) to over 1 MeV for core electrons inelements with a high atomic number. Emission of conduction electrons from typical metals usually requires a few electron-volts, corresponding to short-wavelength visible or ultraviolet light. Study of the photoelectric effect led to important steps in understanding the quantum nature of light and electrons and influenced the formation of the concept of wave–particle duality.Other phenomena where light affects the movement of electric charges include the photoconductive effect (also known asphotoconductivity orphotoresistivity), thephotovoltaic effect, and thephotoelectrochemical effect.Photoemission can occur from any material, but it is most easily observable from metals or other conductors because the process produces a charge imbalance, and if this charge imbalance is not neutralized by current flow (enabled by conductivity), the potential barrier to emission increases until the emission current ceases.
It is also usual to have the emitting surface in a vacuum, since gases impede the flow of photoelectrons and make them difficult to observe. Additionally, the energy barrier to photoemission is usually increased by thin oxide layers on metal surfaces if the metal has been exposed to oxygen, so most practical experiments and devices based on the photoelectric effect use clean metal surfaces in a vacuum.When the photoelectron is emitted into a solid rather than into a vacuum, the term internal photoemission is often used, and emission into a vacuum distinguished as external photoemission.