The photoelectric effect occurs when light above a certain frequency (the threshold frequency) is shone on metals like zinc, this causes electrons to escape from the zinc. The escaping electrons are called photoelectrons.

It was shown in experiments that;

- the frequency of the light needed to reach a particular minimum value (depending on the metal) for photoelectrons to start escaping the metal
- the maximum kinetic energy of the photoelectrons depended on the frequency of the light not the intensity of the light

The above two observation can only be explained if the electromagnetic waves are emitted in packets of energy (quanta) called photons, the photoelectric effect can only be explained by the particle behaviour of light.

The photoelectric equation involves;

- h = the Plank constant 6.63 x 10
^{-34}J s - f = the frequency of the incident light in hertz (Hz)
- &phi = the work function in joules (J)
- E
_{k}= the maximum kinetic energy of the emitted electrons in joules (J)

The energy of a photon of light = hf and the work function (f)is the minimum energy required to remove an electron from the surface of the material. So we can see from the equation above that if the light does not have a big enough frequency (f) so that the photon has enough energy to overcome the work function (f) then no photoelectrons will be emitted.

The above equation can be rearranged into the from y=mx+c

So plotting a graph of frequency (f) on the x-axis and maximum kinetic energy (E_{k}) on the y-axis will give a straight line graph. Where the gradient is the Plank constant (h) and the y intercept is the work function(f), the intercept on the x-axis is the threshold frequency f_{0}.