The process of release of electrically charged particles (usually
→ electrons) as
a result of irradiation of matter by light or other
→ electromagnetic radiation.
The classical electromagnetic theory was unable to account for the
following characteristics of the phenomenon.
Light below a certain threshold frequency, no matter how intense,
will not cause any electrons to be emitted.
Light above that frequency, even if it is not very intense,
will always cause electrons to be ejected.
The electrons are ejected after some nanoseconds, independently of the light intensity.
The maximum kinetic energy of the emitted electrons is a function of the frequency
and does not dependent on the intensity of the incident light.
The classical theory could not explain how a train of light waves spread out over a
large number of atoms could, in a very short time interval, concentrate enough energy
to knock a single electron out of the metal.
In 1905, based on Planck’s idea of → quanta,
Einstein proposed that light consisted of quanta (later called → photons);
that a given source could emit and absorb radiant energy only in units
which are all exactly equal to the radiation frequency multiplied by a constant
(→ Planck’s constant);
and that a photon with a frequency over a certain threshold would have
sufficient energy to eject a single electron. His photoelectric equation is descibed as
(1/2)mu2 = hν - A, where m is the
electron mass, u is the electron velocity, h is Planck’s constant,
ν is the frequency, and A the → work function,
which represents the amount of work needed by electrons to get free of the surface.
See also → photoelectron,
→ photoelectric current,
→ external photoelectric effect,
→ internal photoelectric effect.
See also: → photoelectric; → effect.
