From these observations he concluded that negatively
charged particles were emitted from the zinc plate under the action of
ultraviolet light After the discovery of the electron in 1897, it became evident that the
incident light causes electrons to be emitted from the emitter plate Due
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Dual Nature of Radiation
and Matter
to negative charge, the emitted electrons are pushed towards the collector
plate by the electric field Hallwachs and  Lenard also observed that when
ultraviolet light fell on the emitter plate, no electrons were emitted at all
when the frequency of the incident light was smaller than a certain
minimum value, called the threshold frequency