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Select laws of photoelectric emission?

Laws of Photoelectric emission

1. For a given metal, there exists a certain minimum frequency of incident radiation below which no emission of photoelectrons takes place. This cut-off frequency is called threshold frequency ƒ₀
2. For a given metal and frequency of incident radiation (above threshold frequency), the photoelectric current (that is, photo electrons emitted per second) is directly proportional to the intensity of incident radiation.
3. Above the threshold frequency, the maximum kinetic energy of the emitted photo electrons is independent of the intensity of the incident radiation but depends only upon the frequency of the incident radiation.
4. The photoelectric emission is an instantaneous process. The time interval between the  incidence of radiation and the emission of photo electrons in less than 10^-9s.