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The cutoff frequency is the minimum frequency that is required for the emission of electrons from a metallic surface, due to incident radiation. Below the cutoff frequency, the phenomenon of the photoelectric effect stops altogether.
When electromagnetic radiation is incident upon a metallic surface, if the radiation has a frequency greater than the threshold frequency, a part of the energy of the incident radiation (known as threshold work function) is absorbed by the electrons on the metallic surface; facilitating electronic excitement. The remaining portion (i.e, energy of incident radiation - threshold work function) is transformed into the kinetic energy of the electrons and accounts for their ejection.