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The Photoelectric Effect
The photoelectric effect is the name given to the process in which a photon is completely absorbed by a bound electron, giving the electron enough energy to escape
whatever binding potential is holding it, generating a free electron called a photoelectron. The bound electron is generally either bound to an atom or is held in a crystalline
solid by the collective action of a number of atoms (as in the case of conduction electrons in a metal). Whether or not the photoelectric effect will occur depends on the
binding energy of the electron. The binding energy is the amount of energy that the
electron needs to completely escape from the binding potential.
The photoelectric effect can be observed when light in the visible or UV part of the
spectrum irradiates certain metals. In this case, the energy of the incident photons
may provide enough energy to allow the escape of electrons from a crystalline solid
(the metal). Here the conduction electrons are not bound to a single nucleus, and the
threshold energy for electrons to be emitted is called often called the work function of
the material.
The energy of a photon is related to its frequency by E = h f . Thus the higher the
frequency of the incident light the more energy each photon has, and this energy must
be greater than the work function for the absorption of a photon to result in the electron
gaining enough energy to escape. There is a cut-off frequency below which none of
the photons absorbed will be able to provide bound electrons with enough energy to
escape the binding potential. Thus below the cut-off frequency, no electrons will be
emitted from the metal surface.
Increasing the intensity of the radiation without changing the frequency has the effect of increasing the number of photons striking the material, but does not increase
the energy that each of these photons has. Hence increasing the intensity of the incident light will increase the number of photoelectrons emitted, but only if the frequency
of the light is above the cut-off frequency.
If the frequency of the incident light is greater than the cut-off frequency then there
will be some energy left over after a bound electron absorbs the photon and escapes the
binding potential. This extra energy appears as the kinetic energy of the emitted photoelectron. Some of this kinetic energy may be absorbed by collisions with the crystal
lattice as the photoelectron makes its way to the surface of the metal and escapes, however it is possible that the photoelectron is already at or close to the surface so that it
does not lose any of this kinetic energy. Thus the maximum kinetic energy of photoelectrons emitted by the photoelectric effect is simply the photon energy minus the
binding energy of the metal. The maximum kinetic energy of photoelectrons depends
solely on the frequency of the incident light and not at all on the intensity of this light