The Photoelectric Effect

When light is shone on a photoelectric cell a current starts to flow immediately even if the intensity of light is very low. This occurs due to the photons causing electrons to be emitted from the metal surface and pass to the anode which results in a flow of current.

The photons of light form a one to one interaction with the electrons in the metal which causes emittance to start immediately but only if the frequency is greater than the threshold frequency. This is why the presence of radio waves or infra-red radiation have no effect on the photoelectric cell, despite their obvious presence naturally in the environment. Therefore the number of electrons emitted is proportional to the number of photons so a greater intensity of light gives a greater number of electrons emitted. An increased frequency of the incident radiation gives an increased maximum kinetic energy of the emitted electrons.

The frequency of the incident radiation must be greater that the threshold frequency in order to overcome the work function of the metal and escape its surface. The work function is defined as the minimum energy required for an electron to escape the surface of a metal. The relation between frequency and the work function of the metal is as follows:

hfthreshold =

Where = The work function
h = Planks constant
f = Frequency (Hz)

If the frequency in greater than the work function then the electrons gain kinetic energy. These electrons kinetic energy will vary, however the maximum kinetic energy can be defined using the following equation:

hf = + ½mv2
Where ½mv2 = Ek max = Maximum kinetic energy of the electrons (J)

 



Remember
A greater intensity of light provides more photons per seconds, which gives more electrons per second emitted from the metals, however the maximum kinetic energy of the electrons remains constant.