Explanation: Threshold frequency is the minimum frequency of the incident light which is required for photoelectric effect to take place i.e electrons, get just emitted from the metal surface, with zero kinetic energy. Mathematically : Kinetic energy of photoelectrons, K.E=hν−ϕ (where ϕ is the work function of metal)Let ν and νth be the frequency of incident light and threshold frequency respectively. ∴ K.E=hν−hνth (∵ϕ=hνth)For ν=νth, we get K.E=0Thus at threshold frequency, the kinetic energy of photoelectrons is zero and hence their velocity is also zero.
