Photoelectric Effect, formation and liberation of electrically charged particles in matter when it is
irradiated by light or other electromagnetic radiation. The term photoelectric effect designates several
types of related interactions. In the external photoelectric effect, electrons are liberated from the surface
of a metallic conductor by absorbing energy from light shining on the metal's surface. The effect is
applied in the photoelectric cell, in which the electrons liberated from one pole of the cell, the
photocathode, migrate to the other pole, the anode, under the influence of an electric field.
Study of the external photoelectric effect played an important role in the development of modern
physics. Experiments beginning in 1887 showed that the external photoelectric effect had certain
qualities that could not be explained by the theories of that time, in which light and all other
electromagnetic radiation was considered to behave like waves. For example, as the light shining on a
metal becomes increasingly intense, the classical wave theory of light suggests that the electrons that
absorb the light will be liberated from the metal with more and more energy. However, experiments
showed that the maximum possible energy of the ejected electrons depends only on the frequency of the
incident light, and is independent of the light's intensity.
In 1905, in an effort to explain how the external photoelectric effect occurs, Albert Einstein suggested
that light could be considered to behave like particles in some instances, and that the energy of each
light particle, or photon, depends only on the wavelength of the light. To explain the external
photoelectric effect, he envisioned light as a collection of projectiles hitting the metal. A free electron in
the metal that is struck by a photon absorbs the photon's energy. If the photon is sufficiently energetic,
the electron is dislodged from the metal. Einstein's theory explained many features of the external
photoelectric effect, including why the maximum energy of electrons ejected from a metal is
independent of the intensity of the incident light. According to his theory, the maximum energy of a
dislodged electron depends only on the energy with which a photon strikes the electron; the photon's
energy, however, has nothing to do with the light intensity, because intensity only measures the number
of photons striking the metal. The photon's energy, and thus the maximum possible energy of a
dislodged electron, depends only on the light's frequency. Einstein's theory was later verified through
further experimentation. His explanation of the photoelectric effect, with its demonstration that
electromagnetic radiation can behave like a collection of particles in some situations, contributed to the
development of quantum theory.
Structure of an atom
The photoelectric effect explains that a beam of light can displace electrons in an atom and cause them
to move to a higher level of less energy with loss or emission of energy as light that can be
considered as a particle, as a photon . It was also the first step towards atomic fission and the atomic
bomb. Another explanation is given by this image and text or radioactivity.eu.com
(http://www.radioactivity.eu.com/en/site/pages/PhotoElectric_Effect.htm)
The photoelectric effect occurs in two stages.
1. First, the photon (a) takes out a bound electron in one atom. In the case of gamma photons, it is
usually an electron belonging to the innermost layers L or K (as shown). Then the atom that has lost
one of its inner electrons is left in an excited state.
2. An electron from an outer layer (b) moves to occupy the vacancy left by the ejected electron. If the
ejected electron belonged to the K-shell as in the figure, an X-ray is emitted during this transition.