Further, the uncharged zinc plate became positively charged when it was
irradiated by ultraviolet light Positive charge on a  positively charged
zinc plate was found to be further enhanced when it was illuminated by
ultraviolet light 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