|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:
Lets first study about the nature of these phenomenon:
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.
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:
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.
Answer : wave length = speed of light / velocity