When electrons are emitted from a substance that has absorbed electromagnetic radiation, then photoelectric effect occurs. Albert Einstein, a physicist, was the first to completely define this phenomenon. The photoelectric effect occurs when light of suitable frequency is incident on a metal surface, causing electrons to get emitted. Heinrich Hertz was the first to describe the photoelectric effect, followed by Lenard in 1902. However, neither of the photoelectric effect’s discoveries could be explained by Maxwell’s electromagnetic wave theory of light. Hertz himself did not study the matter since he was confident that the wave theory could explain it. Now let’s understand in detail that what is photoelectric effect and how it can be useful.
Light can be used to push electrons away from a solid’s surface if the conditions are appropriate. The photoelectric effect is a phenomenon that occurs when electrons are ejected from a substance that is photo-emissive. The ejected electrons are termed as photoelectrons, though there is nothing that distinguishes them from other electrons, other than the reason that has caused their production. In terms of mass, charge, spin and magnetic moment, all electrons are similar.
What is photoelectric effect?
It is a phenomenon in which, when light of suitable frequency is made incident on a metal plate then electrons are ejected from it. By apply potential difference, these electrons are moved towards anode, thus producing current. The applied potential must be sufficient so that it can attract electrons to other plate. The word “suitable frequency” is very important in the definition of photoelectric effect as light of any frequency is not capable to make this effect possible. This makes the exact definition of photoelectric effect. This phenomenon proves the particle nature of light.
What are photoelectrons?
The electrons that are ejected from some substance (usually metal surface) due to the exposure of light, are termed as photoelectrons. They are similar to other electrons. The only difference is that light has produced them.
What is photoemissive surface?
A surface that is made up of the atoms, which have plenty of free electrons or loosely bound electrons. So that when any external source of light energy is provided to that surface, it starts to emit those free electrons. Such a surface is called photoemissive.
What is threshold frequency?
This is the minimum value of frequency below which no electrons can be emitted from a surface. Above this value electrons are ejected. Hence it is necessary in photoelectric effect that incident light has frequency greater than threshold frequency. If light having frequency equal to threshold frequency will be incident then ejected electrons can’t travel as they will have no kinetic energy.
What is work function?
Photoelectric effect work function defines the minimum value of energy that is required, in order to eject electrons from some metal surface. Every material has different value of work function. If energy less than work function will be given to a metal surface, no electrons will be emitted. As incident energy is not sufficient to escape electrons from the atoms of that material. It is indicated by “Φ” and has same unit as that of energy.
What is stopping potential?
In the normal case, as from the definition of photoelectric effect, when light of suitable frequency is incident on some metal surface, electrons leave that surface after giving it a positive charge and travel to anode. This will happen when polarity of battery is such that cathode is connected to negative terminal and anode is connected to positive terminal. Now, if the polarity of battery is reversed then, cathode will be at positive potential and anode at negative potential. At lower values of this potential, electrons will keep emitting but their strength will be less definitely. If potential is increased more and more then, a stage reaches when the ejected electrons can’t reach anode. This potential, at which no electron is able to reach anode even value of potential is too high, is called stopping potential. It is indicated by Vo and measured in volts.
Photoelectric effect equation
According to Einstein, light has a particle nature and consists of photons. Each photon carries energy as given below
E = hν
Here “h” is Plank’s constant and “v” is the frequency of light. According to this theory, when photon falls on a metal surface the some part of photon energy is used to escape electron from metal (work function) and remaining energy is transferred into kinetic energy, with which electrons move. Thus
E = W + K.E
hν = W + K.E
If kinetic energy of electrons is to be found then K.E = hν – W
At threshold frequency, work function will be exactly equal to the energy of photon.
Φ = W = hνo
To find the maximum kinetic energy, equation becomes
(1 / 2) m vmax 2 = hν – hνo = h (ν – νo)
Vmax is the maximum kinetic energy of the electron which can be calculated by using stopping potential.
e v0 = (1 / 2) m vmax 2
Thus, Einstein explained the Photoelectric effect by using the particle nature of light.
Photocell and photo-volatic cell are the most common application of photoelectric effect.