3 2  Hallwachs’ and Lenard’s observations
Wilhelm Hallwachs and Philipp Lenard investigated the phenomenon of
photoelectric emission in detail during 1886-1902 Lenard (1862-1947) 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 (Fig 11