(a) It is the phenomenon of ejection of electrons when certain soft metals like potassium, rubidium, caesium, etc. are exposed to a beam of light.
(b) (i) The electrons are ejected from the metal surface as soon as the beam of light strikes the surface of metal.
(ii) The number of electrons ejected is proportional to the intensity or brightness of light.
(iii) For each metal, there is a characteristic minimum frequency (υ0), known as threshold frequency below which photoelectric effect is not observed.
