The Photoelectric Effect

The Photoelectric Effect

The photoelectric effect occurs when light above a particular frequency, known as the threshold frequency, is shone onto a metal like zinc, causing the electrons to escape from the metal. These electrons are known as photoelectrons.

Two main observations are:

  • in order for the photoelectrons to begin escaping from the metal, the frequency of the light has to have reached a specific minimum value (which depends on the metal)
  • the frequency of the light, not the intensity of it, dictates the maximum kinetic energy of the photoelectrons

These observations are only valid if the electromagnetic waves are emitted in packets of energy, or quanta, called photons. How the particles of light behave explains the photoelectric effect.

The photoelectric equation is as follows:

hf = ? + Ek

In which:

  • h = the Plank constant (6.63 x 10-34Js)
  • f = frequency of the incident light in hertz (Hz)
  • ? = the work function in joules (J)
  • Ek = the maximum kinetic energy of the emitted electrons in joules (J)

As the energy of a photon of light is equal to h x f and ? is the minimum energy needed for an electron to be removed from the surface of the material, then if light does not possess a high enough frequency (f) so that the photon has enough energy to overcome ? no photoelectrons are emitted.

It is possible to rearrange the equation above in the form of y = mx + c:

  • hf = ? + Ek
  • hf – ? = Ek
  • Ek = hf – ? which is equivalent to y = mx + c

When a graph of frequency (x-axis) is plotted against maximum kinetic energy (y-axis) a straight line graph is created:

  • the Plank constant is the gradient
  • the work function is the y intercept
  • the threshold frequency (f0) is the x intercept


Collisions of Electrons with Atoms

The electron volt (eV) is a particular amount of energy. It is the energy that would be gained by an electron it if was to be accelerated through a 1 volt potential difference:

  • 1eV = 1.6 x 10-19 joules (J) of energy

In atoms, electrons orbit around the nucleus. It is possible for electrons to exist within an orbit without emitting any energy and to pass between these energy levels. When an electron has enough energy to move from a lower to a higher energy level they are said to be in an excited state. This is known as excitation.

Ionisation, on the other hand, is when an electron gets enough energy to more to infinity.