Radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: electromagnetic radiation, such as radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma radiation (γ).
Electron emission is the process when an electron escapes from a metal surface. Every atom has a positively charged nuclear part and negatively charged electrons around it. Sometimes these electrons are loosely bound to the nucleus. Hence, a little push or tap sets these electrons flying out of their orbits.
The minimum energy required for the electron emission from the metal surface can be supplied to the free electrons by any one of the following physical processes
(i) Thermionic emission: By suitably heating, sufficient thermal energy can be imparted to the free electrons to enable them to come out of the metal.
(ii) Field emission: By applying a very strong electric field (of the order of 108 V m–1) to a metal, electrons can be pulled out of the metal, as in a spark plug.
(iii) Photoelectric emission: When light of suitable frequency illuminates a metal surface, electrons are emitted from the metal surface. These photo (light)-generated electrons are called photoelectrons.
The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The effect has found use in electronic devices specialized for light detection and precisely timed electron emission.
The phenomenon of photoelectric emission was discovered in 1887 by Heinrich Hertz (1857-1894), during his electromagnetic wave experiments. In his experimental investigation on the production of electromagnetic waves by means of a spark discharge, Hertz observed that high voltage sparks across the detector loop were enhanced when the emitter plate was illuminated by ultraviolet light from an arc lamp.
Wilhelm Hallwachs and Philipp Lenard investigated the phenomenon of photoelectric emission in detail during 1886-1902. By a series of experiments, Hallwachs’ and Lenard’s observed that there is a certain minimum frequency, known as threshold frequency, below which no electrons were emitted.
The experimental setup consists of:
As per the wave theory, the maximum kinetic energy of the photoelectron should be affected by the change in intensity. But, the experiments on photoelectric effect showed that maximum kinetic energy doesn’t depend on the change in intensity. So, this was the first inconsistency of the wave theory with the experiments.
Wave theory didn’t talk about the relation between stopping potential and the threshold frequency. It said only increasing the intensity could overcome the stopping potential. Here came the second inconsistency of wave theory with the experimental results.
According to the wave theory, photoelectric effect was not aninstantaneous process, it was time taking. This was the third inconsistency with the experiments.
The above three inconsistencies showed that wave theory of light could not explain the experimentally observed characteristics of photoelectric effect.
Since wave theory could not explain the photoelectric effect, Einstein proposed a particle theory of light for the first time. He said that radiations are made up of specific and discrete packets of energy called as quanta of radiation energy. Each energy quantum has a value equal to hv, where h = Planck’s constant, and v = frequency of incident light. These specific packets of quanta of energy are known as photons.
When a light of frequency(v) (having energyhv ) is incident on a metal surface of work function(Φo), 3 cases could be possible. Case-1-When (hv < Φo), i.e., energy of photon is less than the work function of metal, no photoelectric emission occurs. Case-2- When (hv = Φo), i.e., energy of photon is exactly same as the work function of metal, then electrons get enough energy to just escape the metal surface. Case-3- When (hv > Φo),e., energy of photon is greater than the work function of metal. Then electron, apart from getting energy to escape the metal surface, the remaining energy is provided to the electron as kinetic energy. Mathematically, it can be expressed as: hv = Φo + Kmax Here, Kmax is the maximum kinetic energy of a photoelectron.
Photoelectric effect thus gave evidence to the strange fact that light in interaction with matter behaved as if it was made of quanta or packets of energy, each of energy h ν.
Summarise the photon picture of electromagnetic radiation as follows:
(i) In interaction of radiation with matter, radiation behaves as if it is made up of particles called photons.
(ii) Each photon has energy E (=hν) and momentum p (= h ν/c), and speed c, the speed of light.
(iii) All photons of light of a particular frequency ν, or wavelength λ, have the same energy E (=hν = hc/λ) and momentum p (= hν/c = h/λ), whatever the intensity of radiation may be. By increasing the intensity of light of given wavelength, there is only an increase in the number of photons per second crossing a given area, with each photon having the same energy. Thus, photon energy is independent of intensity of radiation.
(iv) Photons are electrically neutral and are not deflected by electric and magnetic fields.
(v) In a photon-particle collision (such as photon-electron collision), the total energy and total momentum are conserved. However, the number of photons may not be conserved in a collision. The photon may be absorbed or a new photon may be created.
De Broglie proposed that if the radiations could possess dual nature, matters could also possess dual nature. A particle of mass (m), moving with velocity (v) could behave like a wave under suitable conditions. And the corresponding wave related to that matter is called matter wave De Broglie’s wavelength for matter wave is given by: λ = h/p.
Heisenberg uncertainty principle or indeterminacy principle, statement, articulated (1927) by the German physicist Werner Heisenberg, that the position and the velocity of an object cannot both be measured exactly, at the same time, even in theory.
It stated that it is impossible to simultaneously evaluate the precise position and momentum of particle. There is always some probability in predicting the position and momentum of a particle. Mathematically, it can be written as:
(Δx)(Δp) ≥ h/(2π)
The Davisson and Germer experiment demonstrated the wave nature of the electrons, confirming the earlier hypothesis of de Broglie. Electrons exhibit diffraction when they are scattered from crystals whose atoms are spaced appropriately.
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