CBSE NOTES CLASS 12 PHYSICS
CHAPTER 11 DUAL NATURE OF RADIATION AND MATTER
Failure of wave theory and photoelectric effect
- According to the wave theory of light, the free electrons at the surface of the metal (over which the beam of radiation falls) absorb the radiant energy continuously.
The greater the intensity of radiation, the greater is the amplitude of electric and magnetic field. Consequently, the greater the intensity, the greater should be the energy absorbed by each electron.
The maximum kinetic energy of the photoelectrons on the surface is expected to increase with increase in intensity. Also, irrespective of the frequency of radiation, a sufficiently intense beam of radiation (over sufficient time) should be able to impart enough energy to the electrons, so that they exceed the minimum energy needed to escape from the metal surface. A threshold frequency, therefore, should not exist.
These expectations of the wave theory directly contradict observations.
- Also, in the wave theory, the absorption of energy by electron takes place continuously over the entire wavefront of the radiation. Since a large number of electrons absorb energy, the energy absorbed per electron per unit time turns out to be small and it can take hours or more for a single electron to pick up sufficient energy to overcome the work function and come out of the metal. This conclusion is again in striking contrast to observation that the photoelectric emission is instantaneous