Q.How will you define threshold frequency?

The detailed experimental study of how photocurrent responds to intensity, potential difference and frequency, carried out with a dedicated photoelectric-effect apparatus, distills into five compact empirical laws that together fully characterise the photoelectric effect and that any correct theory of it must explain. (1) For a fixed frequency of incident light, the number of photoelectrons emitted per second -- and hence the photocurrent, including the saturation current reached once every emitted electron is collected -- is directly proportional to the intensity of the incident light. (2) The maximum kinetic energy of the emitted photoelectrons is completely independent of the intensity of the incident light; a brighter light produces more electrons, not more energetic ones. (3) For a given metal, the maximum kinetic energy of the photoelectrons is directly proportional to the frequency of the incident light, above the threshold frequency. (4) For a given metal surface, photoelectric emission occurs only if the frequency of the incident light exceeds a certain minimum value called the threshold frequency; below this frequency, no photoelectrons are ever emitted, no matter how intense the light. (5) There is no measurable time lag betwe …