Class 11 Chemistry Structure of Atom Black body radiation and Photoelectric effect

Black body radiation and Photoelectric effect

Particle nature of Electromagnetic radiations :There were two important phenomenon that couldn’t be explained by considering Light with  wave character:

The phenomenon is:

  • Black body radiation
  • Photoelectric effect

Lets first study about the nature of these phenomenon:

  1. Black body radiation: Black body is defined as perfect emitter and absorber of light.

For example, whenever we heat an Iron ball like objects, on heating they become first Red, then Orange, then Yellow and at very high temperature they become White.


According to the electromagnetic theory, the energy is emitted or absorbed continuously. So, the radiations emitted may vary in intensity, but should have same colour. The change in color shows, that wavelength also changes .This indicates non continuous flow of energy. So, it couldn’t be explained by electromagnetic theory. This phenomenon shows, that if we keep on heating the light that it emits. It will show changes in wavelength. That means the wavelength keeps on decreasing but after White there is no further change.

  1. Photoelectric effect

It is the ejection of electrons from metal surface when light of suitable wavelength strikes the metal surface.

The apparatus set to demonstrate this effect is given below:


To check the current flow, the ammeter is introduced in the circuit. The following observations were noticed:

  1. Light of suitable frequency can only eject electrons. Every metal surface has a set value of frequency that can cause ejection and that corresponding frequency is called threshold frequency (vo).
  2. Number of electrons ejected is directly proportional to intensity of light.
  3. Kinetic energy of electron is directly proportional to frequency of light that strikes the surface.
  4. The graphs drawn below shows the above observations:


So, these two-phenomenon Black body and Photoelectric effect couldn’t explain, considering light as a wave. To explain these two phenomena a new concept and new theory was put forward. According to which, light was supposed to possess a particle like character.

Problem 1:

  1. Calculate (a) wave number and (b) frequency of yellow radiation having wave length of 5800 A?

Answer –

  1. Calculation of wave number: wave number = 1/wave length


  1. Calculation of frequency: frequency = velocity(C) /wave length


  1. The Vivid Bharti station of all India Radio, Delhi broadcasts at a frequency of 1368 KHz (kilohertz). Calculate the wave of the Electromagnetic radiation emitted by the transmitter. Which part of the Electromagnetic spectrum does it belongs to ?

 Answer : wave length = speed of light / velocity



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