Characteristics of photoelectric effect: 

(1) For every metal surface there is a limiting frequency of incident radiation below which no photoelectrons are emitted from that metal surface. This frequency, called the threshold frequency, is characteristic of the metal irradiated.

(2) The time rate of emission of photoelectrons in-creases in direct proportion to the intensity, of incident radiation.

(3) The photoelectrons have different speeds at the time of emission ranging from zero to a certain maximum value, which is characteristic for a given metal for a given frequency of the incident radiation. The maximum kinetic energy of the photoelectrons at the time of emission is independent of the intensity but increases linearly with the frequency of the incident radiation.

(4) For incident radiation of frequency greater than or equal to the threshold frequency for a given metal surface, photoelectric emission from the surface is almost instantaneous, even under extremely weak irradiation.