Photoelectric Effect Class 12th

Did you know that Physics is full of exciting experiments that show cases some of unique science effects and photoelectric effect is one of them.

When lightwave of a suitable frequency and wavelength falls on the metal surface, electrons on the metal surface gains energy and is emitted. These emitted electrons are called photoelectrons. This emission of electrons is called as the photoelectric effect. 

So in this article, we will be reading about photoelectric effects and experiments of the photoelectric effect, how it was conducted and how different scenarios and situations can affect the output of the experiment. 

What is photoelectric effect ?

We already know that when light strikes the metal surface, electrons are emitted from the metal surface and this is known as the photoelectric effect. 

Photoelectric effect
Photoelectric effect

The theory of this effect was first proposed in 1887 and later Heinrich Rudolf Hertz gave the experimental verification as well which we will talk about later in this article. 

In the experiment, it was observed that when a surface is exposed to electromagnetic radiation at higher threshold frequency, the radiation is absorbed by the surface and electrons are emitted which are called as a photoelectron. 

Photoelectrons are denoted by the symbol and due to the movement of electrons from one place to another, an electric current is produced known as photoelectric current. 

The photoelectric effect is essential as it helps us to understand the quantum nature of light and electrons. The explanation of the photoelectric effect was successfully given by Albert Einstein. He concluded that light travels in the form of the quantized packet and therefore this effects occurs because electrons present on the surface gets energy in the form of packets. 

So when electrons come in contact with electromagnetic radiation of higher threshold frequency, energy packets absorbed supplies enough energy for an electron to leave the metal surface and emit out. 

Principle of the photoelectric effect:

So, every experiment is based on some scientific theory or principle. In the same way, the photoelectric effect is based on the law of conservation of energy.

The energy transferred from the quantized packet gets absorbed by electrons present on the metal surface and provide them with enough energy to emit out of the surface. 

So in the end, if you look at the bigger picture of the whole process, the energy is just conserved and transferred from one place to another.  

Minimum condition required for photoelectric effect:

It's not like, whenever light falls on the metal surface an electron is emitted and you will encounter a photoelectric effect. There are some requirements for electron emission and photoelectric effects. 

The minimum requirements for photoelectric effects are 1. Threshold frequency 2. Threshold wavelength and 3. Work function

So let's look into each of them specifically to understand what they are.

Threshold frequency:

It is the minimum frequency of incident light required to eject an electron from the metal surface i.e minimum frequency of light needed to present for photoelectric effect and emission of electron from the metal surface. The threshold frequency is constant for a specific metal but it may differ from metal to metal.

If y = frequency of incident light and y' = Threshold frequency. Then, if (y) incident frequency is less than (y') threshold frequency (y < y') then there will be no emission of photoelectrons and therefore no photoelectric effect will occur.

Whereas if (y) incident frequency is greater than (y') threshold frequency (y > y') then the photoelectric effect will take place and photoelectron will come out of the metal surface with some kinetic energy in it.

Now, this was the case when the frequency of incident light is either more or less than the threshold frequency but what will happen when both are the same ? 

In cases like this when the frequency of incident light is the same as the threshold frequency (y = y') the photoelectric effect will still occur but photoelectron will just get ejected from the metal surface but will not have any kinetic energy to move further.

Threshold wavelength: 

It is the maximum wavelength of incident light above which "no photoelectric effect" will take place. It means if the wavelength of the incident light is above the threshold wavelength, the photoelectric effect will not take place and no emission of electrons will be there and if the wavelength is under the threshold wavelength, the photoelectric effect will take place and the electron will get emitted.  

If λ = Wavelength of Incident light and λ' = Threshold wavelength. Then, if (λ) wavelength of incident light is less than (λ') threshold wavelength (λ < λ') then the photoelectric effect will take place and the emitted electrons will have some kinetic energy as well.

Whereas if (λ) wavelength of incident light is more then (λ') threshold wavelength (λ > λ') then there will be no electron emission and photoelectric effect will not take place.

Now in the same way if (λ) wavelength of incident light is equal to the (λ') threshold wavelength (λ = λ') then you will notice that electron is getting emitted and the photoelectric effect is there but the emitted electron will not have any kinetic energy i.e kinetic energy of emitted electron will be zero.

Work function: 

The work function is the minimum energy required to remove an electron from the metal to the point in the vacuum just outside the metal surface. 

It means if the energy (E) of the incident photon is more than the work function (Φ) of the metal
(E > Φ), then the electron will be emitted out of the metal surface and the photoelectric effect will take place. In this case, the emitted photoelectron will have kinetic energy as well.

In the same way, if the energy (E) is less then the work function (Φ) then no photoelectric effect will take place and electrons will not be emitted, but if energy (E) of the incident photon is equal to work function (Φ) of the metal (E = Φ) then an electron will be emitted but without any kinetic energy and photoelectric effect will take place.

Photoelectric effect formula:

As I told you before, that Einstein explained the photoelectric effect and according to it: 
The energy of photon = energy required to remove an electron from metal + kinetic energy of the emitted electron. 
i.e hv = W + E

where, h = plank's constant, v = frequency of incident photon, W = work function and E = kinetic energy of emitted electron.

Laws of the photoelectric effect:

Now as you know about photoelectric effect and the minimum condition required for the photoelectric effect. It's now time to understand certain laws of the photoelectric effect to understand when and how it works. 

I know you may already have understood these laws but the main four laws of photoelectric effect are as follow: 

  1. The photoelectric emission is an instantaneous process. 
  2. The kinetic energy of a photoelectron depends on the frequency of incident light
  3. For a light of any frequency above a threshold frequency, the photoelectric current is directly proportional to the intensity of light. 
  4. If the frequency of incident light is under threshold frequency, the photoelectric effect will not take place no matter how high the intensity of light is. 

I have explained the photoelectric effect experiment in a whole different article. Click on the link above to read that article and understand the experiment. 

Characteristics of the photoelectric effect:

Some characteristics of the photoelectric effect you need to know are as follows: 
  • The threshold frequency is different for different materials. 
  • The kinetic energy of photoelectric is directly proportional to light frequency. 
  • The process of electron emission in the photoelectric effect is instantaneous. 
  • The stopping potential is directly proportional to the frequency
  • The photoelectric effect produced in photoelectric current is directly proportional to light intensity. 

Factors affecting the photoelectric effect:

You now know almost all the details about the photoelectric effect, except one "Dependency of photoelectric effect" as this effect, is only possible if its minimum requirements are fulfilled. 

What will happen if any one of them changes slightly ? How will it impact on our final conclusion ? This is what we are going to read about ahead.

So the photoelectric effect only depends on 3 main factors: 1. The intensity of radiation 2. The potential difference between metal plates and 3. Frequency.

Let's see how all these factors affect the photoelectric effect: 

Effect of intensity of radiation on the photoelectric effect:

In this case, the potential difference between the metal plate and collector is kept constant and intensity is changed. 

The conclusion of the following setup shows us that photoelectric current and intensity of light is directly proportional to each other. Photoelectric current shows us the amount of photoelectrons ejected from a metal surface per second. 

Effect of potential difference between metal plates on the photoelectric effect:

Well I have already covered this particular portion in one of my different articles here in-depth, so do read it as well for better understanding. 

Long story short, in this scenario, the potential difference between metal plate and collector kept changing and varied whereas the intensity of radiation and frequency was kept constant

In the following setup, with the increase in potential difference, the photoelectric current is also increased but after reaching the maximum current value (saturation current) there is no further increase in photoelectric current even after increase in potential difference. 

Effect of frequency on the photoelectric effect:

In the following case, the frequency kept changing and the intensity of light was constant. The kinetic energy of emitted photoelectron is directly proportional to the frequency of incident light. 

Application of photoelectric effect:

Learning all those different scientific theories is of no sense if they don't have any benefit but learning photoelectric effect does. 

They may not directly benefits you but indirectly it does because this effect is used in many industries on which you are directly dependent such as electricity. 

So here are some applications of photoelectric effect:

  • The photoelectric effect is used in photodiodes, light meters, and phototransistors
  • Used is photocell that is used in solar panels to generate electricity. 
  • Used in scintillators which emits light whenever it detects some kind of radiation from either test lab or cosmic sources. 
  • In digital cameras to detect and record light which can be detected by the photoelectric sensor present in them. 
  • Used in X-ray Photoelectron Spectroscopy also known as XPS where X-ray is used to irradiate a surface and measure the kinetic energy of photoelectron. Surface chemistry can be obtained such as elemental composition, compounds empirical formula, chemical state, and elemental composition. 

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