i) The number of electrons emitted per second is directly proportional to the intensity of the radiation.

ii) The maximum kinetic energy of the electrons emitted increases with the frequency of the radiation.

iii) There is a minimum frequency below which no emission occurs.

A graph of K.E._max of photo-electrons against frequency of radiation has the following form.

The minimum frequency for photo-electric emission is called the threshold frequency, f_o.

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