The emission of electrons from a metal surface when it is irradiated with light is called the photoelectric effect. The electrons emitted in this process are called photoelectrons. The kinetic energy of these photoelectrons is given by the equation:

$$KE = hf-\Phi$$

Where h is Planck's constant, f is the frequency of the incident light, and Φ is the work function of the metal. The work function is a characteristic of the metal and represents the minimum energy required to remove an electron from the metal.

From the equation it is clear that if the frequency of the incident light is less than the threshold frequency, then the kinetic energy of the photoelectrons will be negative and they will not be emitted. This explains why for a particular metal electrons are emitted only when the frequency of the incident radiation is greater than a certain value.