Page 99 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch08 Elementary Physical Chemistry

84 Elementary Physical Chemistry

The prism separates the light, which is a mixture of several frequencies
(colors), thus providing information on the distribution of the frequencies
(or colors). The theoretical prediction of the way frequencies should vary
with temperature did not agree with the experimental results.
Classically, it was assumed that the electromagnetic field is made up
of a collection of oscillators. If an oscillator is excited to a frequency ν
it will emit radiation of that frequency. Planck noticed that if he would
disregard one of the most sacred rules of science, namely that energy varies
continuously, he could explain the black-body radiation.
Planck then postulated that the oscillator energies varied discontinu-
ously — that the energies are discrete, not continuous. Basically, Planck
suggested that in the black-body experiment, energy can be absorbed from
the heat source only in discrete units, called quanta.

8.2.2. Photo-Electric Effect
When a beam of light shines on a metal, it may cause electrons to be ejected
from the metal. This is referred to as the photo-electric effect. However,
there is a minimum (or threshold) frequency (different for different metals)
below which electrons are not ejected, regardless of how intense the light
beam is.

Note: If light consisted of waves, as was generally assumed in those
days, there was no reason why there ought to be threshold frequency.
As long as the beam is sufficiently intense, it should emit electrons
regardless of frequency.

Could Planck’s ideas shed some light on this problem? Planck’s
suggestion of 1900 was essentially forgotten until Einstein, a patent clerk
at Bern, Switzerland, published a paper in 1905, which not only revived
Planck’s ideas but plunged science into a state of turmoil that lasted for
more than a quarter of a century.
Einstein felt that Planck’s explanation of the black-body radiation,
though revolutionary, was incomplete. While Planck’s theory clearly sug-
gested that energy absorbed from the heat source by the black-body must do
so in quanta, not much was said about the fate of the energy of the emitted
radiation. It was generally assumed that the emitted radiation would obey