Explanation

(d) The emission of photoelectron takes place only, when the frequency of the incident light is above a certain critical value, characteristic of that metal. The critical value of frequency is known as the threshold frequency for the metal of the emitting electrode.
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Suppose that when light of certain frequency is incident over a metal surface, the photo-electrons are emitted. To take photoelectric current zero, a particular value of stopping potential will be needed. If we go on reducing the frequency of incident light, the value of stopping potential will also go on decreasing. At certain value of frequency $v_{0}$, the photoelectric current will become zero, even when no retarding potential is applied. This frequency $v_{0}$ corresponds to the threshold for the metal surface. The emission of photo-electrons does not take place, till frequency of incident light is below this value.