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 1)