How is electron emitted in photoelectric effect experiment


In a photoelectric experiment, it's actually possible to have current in a circuit because of electron emission.

When a light wave with high frequency than threshold frequency falls on a surface, electrons present on the surface gets enough energy to leave the surface.

Don't worry read further and we will clear all your queries from the basics about everything from electron emission to threshold frequency.

Electron emission in photoelectric effect experiment:

The photoelectric effect experiment is a cool experiment where current is flown in the circuit with the help of electron emitted in the photoelectric effect. Well if you don't know much about this experiment I have an article where I have explained the photoelectric effect and what is the effect of potential in observation. Click on a photoelectric experiment to read that article.

In a photoelectric experiment, a light wave of suitable frequency which is higher or equal to threshold frequency of metal is to be fallen on a metal surface which provided electron with enough energy to leave the outermost orbit of atom and that's why the electrons are emitted and current flows in the circuit. 


What is the photoelectric effect ?

So let's start with the basis of the photoelectric effect and understand what is photoelectric effect? After that, we will learn why electrons are emitted in the photoelectric current. 
Photoelectric effect
Photoelectric effect
When a light wave of suitable frequency hits the metal surface, an electron on the surface gets energized and leaves the metal surface. This emitted electron is called photoelectron and this process of emission of the electron due to the striking of lightwave on a metal surface is called photoelectric effect.


What is the emission of an electron ?

So emission of electron refers to leaving of an electron from the surface of the object. In a solid, when electron present at the surface gets energised can leave the surface and becomes a free electron.

Well, if you want to understand why electrons leave surface after getting energized? Don't worry I got your back. So first let's understand about Bohr's model, threshold frequency and work function which will clear all your queries :)

Bohr's model of an atom:

So there are many models of an atom present but with some limitations of there own. Well, Niels Bohr also proposed a model of atom known as Bohr's model. We are not going to learn about Bohr's model in detail because our main query is to find out why electron get emitted from the surface after getting energized?

Well, let's first understand the main point of Bohr's model:
  • Electrons revolve in a definite circular orbit around the nucleus.
  • Each orbit has fixed energy and while revolving around these orbit electron can neither gain nor lose energy. 
  • When an electron moves from high energy orbit to low energy orbit it's need to lose some energy to fix in low orbit and therefore it radiates energy. 
  • Same way when an electron gains energy it jumps from lower orbit to higher orbit.
Electron Emission
Electron Emission
So now you may have got your answer, I think!! Well, electrons which get emitted from the surface in photoelectric effect are the one in the highest energy orbit of atom and when energy is provided to them there is no space left for them to jump and they become free electron.

Threshold frequency:

Well, you now understand about electron emission and why electron from the surface is emitted after gaining energy but it's not like that falling of light will emit electron every time. There are some standards and conditions which are needed to fulfil. Not any and every light can emit an electron from the surface.

You all know well that all light waves have some certain frequency and threshold frequency is the minimum requirement of an electron to get emitted from the surface.

If the light is less then the threshold frequency is to fall on the surface, that light wave is not been able to emit and electron whereas if light wave with a frequency greater than or equal to threshold frequency falls you will see electron emission from the surface.

So threshold frequency is the minimum amount of frequency needed to emit an electron from the surface.

P.s:- Every material has a different threshold frequency, therefore, the different frequency of light waves is needed to emit electrons from different surfaces.

Work function:

So now you know the answer to why electron from the surface is emitted after getting energized and why not every light wave can remove an electron?

But there is one more thing which is left, how much energy is needed to remove an electron from the surface?

Well, your answer is WORK FUNCTION just like threshold frequency is a minimum frequency of lightwave needed to be able to remove an electron from the surface, the work function is also kind of similar.

When a light wave hits the surface, energy gets transferred to electron and it gets emitted and you know that electron leaves the outermost orbit and becomes free-electron after leaving the surface.

It does need some energy to leave the outermost or highest energy orbit of atom and the amount of energy required by an electron to just leave the orbit and enter free space is called work function.

Work function is just the energy required to leave the outermost orbit nothing else. If an electron is provided with sufficient energy that it can leave orbit as well as can travel a certain distance also then that is not counted as work function.

P.s: Work function is just the energy required to make electron free from orbit and surface.
Every metal has different work function and here is the table in which the work function of 20 different metals is present:

 The work function of different metals
The work function of different metals

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