Photoelectric effect

When light of a short enough wavelength hits a metal, electrons are released. This is called the photoelectric effect. Classically, the electrons released (current) should be proportional to the intensity of the light (total energy delivered); however, this is not the case.

It is observed that long wavelength light does not release any electrons. This makes no sense classically. Moreover, even a very low intensity of short wavelength light releases some electrons.

Einstein explained the photoelectric effect by saying that light consists of quantized energy units (photons) with energy $E=h\nu$ (given by the Planck-Einstein relation). Electrons are bound to the metal with a binding energy $W_0$ called the work function. In order for electrons to be freed, the incident photons need to carry at least $E_p \ge W_0$.

By conservation of energy, $E_p = W_0 + K$ where $K$ is the kinetic energy of the electron.

We see that light below frequency $\nu_0$ will not overcome the binding energy. $\nu_0$ and $W_0$ depend on the metal, but the slope $h$ is always the same.

The opposite process, using electrons to emit photons, is X-ray production.