Explain -"for photoelectric emission frequency of incident radiation should be greater than threshold frequency".

Explanation

For different metals,value of threshold frequency is different.
Different photosensitive substance have different response to incident radiation.
Selenium is more sensitive than zinc and copper ifferent response to lightt of different wavelength `to` ultraviolet light gives rise to photoelectric effect in copper whereas green or red light does not.
If frequency of incident radiation exceeds the threshold frequency,the photoelectric emission start instantaneously even if intensity of incident radiation is less.
Emission start in time of `10^(-9)`s or less.