Geoscience Reference
In-Depth Information
the behavior of black-body radiation at high frequencies (short wavelengths) but fails to
predict accurately the behavior of black-body radiation at low frequencies (longer wave-
lengths). Max Planck, who is considered by many to be the founder of quantum mechan-
ics, discovered that the intensity of electromagnetic radiation emitted by a black body is
dependent on
both
the frequency of the radiation (the color of light) and the temperature of
the emitting body. Planck (
1900
) stated that the energy of the charged oscillators in a black
body must be
quantized
and that electromagnetic energy can be emitted only in a quantized
form. This is to say that the energy (
E
) can only ever be a multiple of an elementary unit
given by the equation
E
=
ν
(1.1)
where
h
is Planck's constant, and
ν
(the Greek letter nu) is the frequency of the oscillator.
This later became known as the Planck postulate. The assumption that electromagnetic
radiation (light) is quantized allowed Planck to derive a mathematical formula that could
be applied to the
entire
electromagnetic spectrum, unlike Wien's Law, which was true
only for short wavelengths (UV-Vis). At the time, Planck believed that the quantization of
energy applied only to the tiny oscillators related to matter under investigation and made
no assumption that light itself is quantized. Planck's concern was one of solving the math-
ematical problem highlighted earlier by Wien rather than proposing a fundamental change
in the understanding of the world. Despite this, Planck's postulate was to help transform
our understanding of the world and universe in which we exist.
The photoelectric effect is the phenomenon whereby electrons are emitted from mate-
rial, such as metals, nonmetals, liquids, and gases as a direct consequence of their absorp-
tion of energy The achievements of Hertz in observing the photoelectric effect were very
important as it paved the way for Johann Elster and Hans Geistel to pioneer the reliable
production of photoelectric devices at the turn of the 20th century. These photoelectric
devices could accurately measure the intensity of light far beyond the capability of the
human eye. In 1902 before the discovery of the electron, Philipp Eduard Anton von Lenard
observed that the energy of individual emitted particles from a cathode ray increased with
the frequency of the light rather than the intensity of light (Philipp Lenard - Biography). At
the time this postulate was in direct conflict with James Clerk Maxwell's electromagnetic
wave theory, which predicts that the energy of the electromagnetic wave would be propor-
tional to the intensity of the radiation as opposed to frequency. In 1905, Albert Einstein
described light as being composed of discrete quanta (what we now know as photons),
rather than as a continuous wave of energy. Using Max Planck's theory of black-body
radiation, Einstein theorized that the energy in each quantum of light was equal to the fre-
quency multiplied by a constant (later named Planck's constant). Therefore a photon above
a threshold frequency has the required energy to eject a single electron. This work led to the
theory of unity, which took into account that both electromagnetic waves and subatomic
particles possessed properties both of particles and electromagnetic waves, the so-called
wave-particle duality (Einstein,
1905
).
