question_answer 137)

  What is photoelectric effect? State the result of photoelectric effect experiment that could not be explained on the basis of laws of classical physics. Explain this effect on the basis of quantum theory of electromagnetic radiations.

Answer:

  Experiments show that electrons are ejected from the surface of certain metals (like Cs, K and Rb having low ionisation energies) when they are illuminated by light or any other radiation of suitable frequency or wavelength. This phenomenon is known as photoelectric effect. The electrons so emitted are called photoelectrons. The experiment is carried out in a photoelectric cell which consist or an evacuated glass tube fitted with two electrodes connected through an outer circuit as shown in figure given below. The cathode is coated with a metal such as caesium. When light of sufficiently high energy, i.e., high frequency strikes the metal, the electrons are ejected as shown in the figure and move towards anode and a current flows through the circuit.  Absorbed quantum = Threshold + Kinetic Energy \[E\,\,\,\,\,\,\,\,\,\,=\,\,\,\,\,\,\,\,\,\,\,{{E}_{0}}+KE\] \[hv\,\,\,\,\,\,\,\,\,=\,\,\,h{{v}_{0}}+\frac{1}{2}m{{\upsilon }^{2}}\] \[h\frac{c}{\lambda }=h\frac{c}{{{\lambda }_{0}}}+\frac{1}{2}m{{\upsilon }^{2}}\] Maxwell theory of light explained its rectilinear propogation, reflection, refraction, interference, diffraction, polarisation etc. quite successfully but it failed to explain black body radiation and photoelectric effect.