When ultraviolet rays fall on the negative plate, the electrons emitted from the plate are attracted by the positive plate so that the circuit is completed and current flows. But if the rays fall on the positive plate, the electrons emitted from this plate do not reach the negative plate because the electrons are negatively charged. Hence, the circuit is not completed and the current does not flow.
The phenomenon of emission of electrons from metals under the effect of light is called ‘photoelectric effect’.
The electrons emitted by photoelectric effect are called ‘photoelectrons’ and the current produced is called ‘photoelectric current’.
For this phenomenon the light of short wavelength (or high frequency) is more effective than the light of long wavelength (or low frequency).