We now apply a negative (retarding)
potential to the plate A with respect to the
plate C and make it increasingly negative
gradually When the polarity is reversed,
the electrons are repelled and only the
sufficiently energetic electrons are able to
reach the collector A The photocurrent
is found to decrease rapidly until it drops
to zero at a certain sharply defined,
critical value of the negative potential V0
on the plate A For a particular frequency
of incident radiation, the minimum
negative (retarding) potential V0 given to
the plate A for which the photocurrent
stops or becomes zero is called the cut-
off or stopping potential