In this particular article History of the photoelectric effect and its explanation, we are going to discuss the history of the photoelectric effect and its explanation.
History of the photoelectric effect and its explanation
In this particular article History of the photoelectric effect and its explanation, we are going to discuss the history of the photoelectric effect and its explanation. The photoelectric effect was initially observed by Hertz in 1887. according to Hertz when the light radiations of lower wavelength (visible or UV light) falls on the metallic plates then the electrons are emitted from the metallic plate. Hallwachs also observed that ultraviolet radiation fall on the negatively charged zinc plate then it becomes neutral. The reason for this is when radiations fall on zinc plate then it loose its outermost electron and becomes positive.
J.J. Thomson and Lenard in some of their experiments observed that ” when the light of a certain frequency incident on the surface of a metal, then consequently electrons are ejected from the metal. This phenomenon is known as the photoelectric effect. And those electrons which are ejected by the surface of the metal are called photoelectric electrons. Only a few numbers of metals are known which shows this photoelectric effect under the action of visible light. But many more metals shows this effect under the action of more energetic U.V. light.
Some important observations
After making careful studies of photoelectric effect under different conditions the important observations made are-
- Every metal has a certain minimum frequency of incident light to eject electrons. This minimum frequency is known as threshold frequency. It is denoted by (ν₀). Radiations of the frequency lower then this, can not pull out the electrons out from the surface if the metal. No matter how long it’s falling on the surface of the metal.
- As the light falls on the surface of the metal immediately electrons are ejected. I.e. the photoelectric effect is an instantaneous process. The i.e. energy is not accumulated.
- The kinetic energy of the emitted electrons is directly proportional to the frequency of incident light. But on the other hand, it does not depend on the intensity of incident light. According to this the kinetic energy of the photoelectrons ejected by red radiation is less than the electrons ejected by ultraviolet radiations. If the metal is equally photosensitive for both types of radiations.
- The number of electrons from the metal surface increase with the increase in the value of the intensity of incident radiation. In other words, We can say that the number of ejected electrons is proportional to the intensity of light falling on it.
explanation of the photoelectric effect
Photoelectric effect cannot be explained on the basis of the classical way theory of light. Einstein explained that the photoelectric effect could be explained if light consisted of discrete particles. Or in other words, it should be in form of bundles of energy, i.e. photon. It should be noticed that each photon have an energy of hν. When a photon of frequency ν ( and energy hν) strikes on the surface of the metal it ejects out electrons from it.
In this process, (1) A certain amount of energy is used up in ejecting out the electron from the metal. This minimum amount of energy which is used to eject out the electron out from the surface is called, work function(W₀) of the metal. (2) Some work is to be done to eject out the electron from the metal plate the minimum amount of work is known as work function (w₀) of the substance (metal). The remaining energy which is equal to the difference of the energy of the incident photon and work function of the metal would be given as kinetic energy to the ejected electron. Thus,
hν = W₀+1.mv²/2 (1)
Where v is velocity and m is mass of the ejected electron respectively. And also W₀ depends upon the nature of metal, not on the incident photon.
The minimum amount of energy required to remove the electron from the metal plate is known as threshold energy (E = W₀).
It is clear from the above that W₀ accounts for the threshold frequency. Also by the relation hν =W₀ thus equation (1) can be written as-
hν = hν₀ + mv²/2
and, mv²/2 = hν-hν₀ = h(ν-ν₀) (2)
Here equation (2) is known as Einstein’s photoelectric equation. Also From equation (2), it is clear that if the energy of the incident photon (hν) is less then the minimum required energy. Then no emission of the electron can take place from the metal surface.
If the kinetic energy of the ejected electron is plotted against the frequency (ν). Then consequently a straight line with slope equal to Planck’s constant (h) and intercept equal to hν is obtained. This is one of the best methods to determine the value of Planck’s constant.
The Conclusion of History of the photoelectric effect and its explanation
The conclusion of the History of the photoelectric effect and its explanation is that we have discussed the history of the photoelectric effect along with its explanation.