If a quantum of radiation with an energy greater than the work function
E_{o}, and therefore a frequency greater than f_{o}, falls on a surface an electron will
be emitted with some kinetic energy and escape from the surface.

The kinetic energy of
the electron is then just the difference between the energy of the quantum and the work function of
the metal.

This can be expressed in the following equation - called Einstein's photoelectric
equation.

This is shown in the diagram.

Calculate the maximum kinetic energy of an electron that is emitted from a caesium surface when light of wavelength 210 nm falls on it. Work function for caesium = 1.88 eV.

Work function = 1.88x1.6x10

Frequency of incident radiation = 3 x 10

Kinetic energy = hf - hf

If we put a collecting electrode in front of the
emitting surface in a vacuum we can detect the photoelectrons as a small current. Changing the
frequency of the radiation will not have any effect on the current as long as it is bigger than
f_{o} but increasing the intensity will increase the current.

Now if we make the collecting
electrode slightly negative compared with the emitting surface the electrons will find it difficult to get
to it and electrons will only do that if their energy is greater than the "height" of the potential barrier.

They will be detected if ½ mv

where V is the potential difference between the emitting surface and the collecting electrode.

Therefore Einstein's equation can be written as:

If V is increased so that no more electrons can reach the detector this value for the potential is called the