Question

Explain the observations made by Hertz and Lenard about the phenomenon of photoelectric
emission.

Answer 1

Observations made by Hertz and Lenard:

a) The phenomenon of photoelectric emission was discovered in 1887 by Heinrich Hertz.

b) During his experimental investigation on the production of electromagnetic waves by
means of a spark discharge, Hertz observed that high voltage sparks across the detector
loop were enhanced when the emitter plate was illuminated by ultraviolet light from an
arc lamp. 

c) 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.

d) As soon as the ultraviolet radiations were stopped, the current flow also stopped. These observations indicated that when ultraviolet radiations fall on the emitter plate C,
electrons are ejected from it which are attracted towards the positive collector plate A by
the electric field.

e) The electron flow through the surface of emitter caused current in the external circuit.

f) Hallwachs and Lenard studied how this photocurrent varied with collector plate
potential, frequency and intensity of incident light.