Define the term work function of a metal.

Hint: In the photoelectric effect, when we shine a light of certain frequency on the metal, some part of the energy of the radiation is used in freeing the electron from the atoms. The electrons are held in an atom by the pull of the nucleus which must be overcome in order to remove the electron from the atom.

Complete step-by-step answer:
The work function of a given metal can be defined as the amount of energy which is required to pull out an electron from an atom. This energy is equal to the energy of the electron revolving around the nucleus.
In photoelectric effect, we shine a light of certain frequency on a metal. The light supplies energy to the atom. The minimum amount of energy which is able to just pull out the electron from the atom is known as the work function of the metal.
Additional information:
In photoelectric effect, we have a radiation of certain wavelength or frequency incident on a metal surface. The kinetic energy of emitted photoelectrons is equal to the difference between incident energy and the work function of the metal.
$\therefore \dfrac{1}{2}m{{\rm{v}}^2} = h\nu - W$
1. Photoelectric current in a photocell increases with the increase in the intensity of the incident radiation.
2. The stopping potential (\[{V_0}\]) varies linearly with the frequency ($\nu $) of the incident radiation for a given photosensitive surface with the slope remaining the same for different surfaces.
3. Maximum kinetic energy of the photo-electrons is independent of the intensity of incident radiation.

Note:The photoelectric effect cannot take place if the energy of the incident radiation is less than the work function of the metal. The part of the incident energy is used to pull out the electron and rest is converted into the kinetic energy of the emitted electrons.