Explain Hallwach's and Lenard's observation on photoelectric effect.
Define :
a.work function
b. Threshold frequency
c. Stopping potential

Explanation

Hallwach.s and Lenard conducted a detailed study of the phenomenon of photoelectric emission.
Lenard observed that when ultraviolet radiations were allowed to fall on the emitter plate of an evacuated glass tube enclosing two electrodes (metal plates), current flows in the circuit. As soon as the ultraviolet radiation is stopped , the current flows also stops. Thus, light falling on the surface of the emitter causes current in the external circuit.
Hallwach.s observed that the uncharged zinc plate became positively charged when it is irradiated charged zinc plate gets enhanced when it is illuminated by ultraviolet light. From the experimental observations he conclude that zinc plate emits negatively charged particles under the action of ultraviolet light.
Definitions:
a. Work function :The minimum energy required for an electron to excape from the metal surface is called the work function.
Threshold frequency : Threshold frequency of a metal is the minimum cut-off frequency of incident light below which no photoelectric emission takes place irrespective of intensity of incident light.
c. Stopping potential : Stopping potential of a photo sensitive metal is defined as the minimum negative potential applied to the collector at which the photoelectric current just drops zero.