Laws Governing the Photoelectric Effect

When a metal surface or a metal body is exposed to the energy of light of preferred frequency, then emission or ejection of electrons occurs from the metal surface, this phenomenon is known as the photoelectric effect. Photoelectrons are the electrons that are ejected by the effect of incident light. Photoelectric currents are currents that are composed of photoelectrons.

Laws of the photoelectric effect 

Photoelectric effect laws are as follows:-

• 1st Law of the photoelectric effect

 The intensity of the incident radiation is proportionate to the emission of the number of photoelectrons per second, which simply means that if the number of electrons emitted per second increases in number then the intensity of the incident radiation also increases, and if the amount of electrons emission decreases then the intensity of the incident radiation also decreases. The graph shown below shows the emission of photoelectrons per second, concerning the intensity of light.

• 2nd Law of the photoelectric effect

 When the frequency of the emission of light is equivalent to or becomes more than the metal’s work function, then the electrons get emitted by the surface of the metal. 

hf  φ

• 3rd Law of the photoelectric effect

 The intensity of the incident radiation is proportionate to the saturation current. So, if the intensity of the incident radiation is increased then the saturation current also increases, and if the intensity of the incident is decreased then the saturation current also decreases.

From the above graph, it can be seen that I3 > I2 >I1. The saturation point of I3 is the highest as the intensity of the incident radiation is also highest and the saturation point of I1 is lowest as the intensity of the incident radiation is also lowest.

• 4th Law of the photoelectric effect

 There is independence between the stopping potential and the intensities of the incident radiations, i.e. the intensity of the incident radiation varies but the stopping potential remains similar for all intensities. The graph below shows this

The stopping potential (V0) is similar for all the intensity of the incident radiation (I1, I2, and I3)

• 4th Law of the photoelectric effect

 The value of saturation current is not dependent on the frequency i.e. they are independent of each other. Their frequency differs but their saturation point remains the same for all. The graph given below shows the saturation current with different frequencies.

• 5th Law of the photoelectric effect

 As the frequency is increased then the stopping potential will also increase negatively. The graph below shows the increase in frequency along with the increase of stopping potential negatively but the saturation point remains similar for all.

• 6th Law of the photoelectric effect

 After the minimum cut-off frequency, the photoelectric stopping potential increases in a linear manner along with the frequency. This simply means that after a certain cut-off frequency the stopping potential along with the frequency increases linearly.

• 7th Law of the photoelectric effect

 For producing a photoelectric effect the minimum frequency of the incident light that is required for the ejection of the electron is called threshold frequency where the minimum frequency of incident light below which no emission takes place. 

• 8th Law of the photoelectric effect

The phenomenon of the photoelectric effect is a spontaneous or random process that does not lag or stop in between. 

Applications or uses of the photoelectric effect –

a) For the working of phototransistors, light meter photocopies, and photodiodes photoelectric effect is used.

b) Photoelectric effect is also used in photo-cell; it is the most important use of photoelectric effect and is generally found in solar panels. This work on the basic principle of the photoelectric effect that is when the incident of light strikes the metal surface, the ejection of electrons occurs which gives rise to producing current.

c) Photoelectric effect is used in photomultiplier tubes, which works on the principle of converting the intensity of light to form an electric current.

Conclusion

It is concluded that the photoelectric effect is defined as the ejection or emission of electrons from the metal surface when the incidence of light that contains enough energy falls in a metal. The electrons that are emitted are known as photoelectrons. There are several photoelectric effect laws.