Q.Define work function of a metal. Give its unit.

The work function is the least energy an electron at the Fermi level needs to escape a metal's surface; it is measured in joules or electron-volts.

1. Definition. In a metal, free (conduction) electrons are held inside by the attractive forces of the positive ion cores. To remove the most weakly bound electron from the metal surface into vacuum (with zero kinetic energy outside), a minimum amount of energy must be supplied. This minimum energy is called the work function of the metal, denoted $\phi$ or $W_0$.

2. Relation to threshold frequency. If $\nu_0$ is the threshold (minimum) frequency of incident light that can just cause photoemission from the metal, then

$\phi = h u_0$

where $h$ is Planck's constant. Equivalently, in terms of threshold wavelength $\lambda_0$,

$\phi = \dfrac{hc}{\lambda_0}$

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