Planck's Hypothesis

In 1900 Max Planck proposed a formula for the intensity curve which did fit the experimental data quite well. He then set out to find a set of assumptions -- a model -- that would produce his formula. Instead of allowing energy to be continuously distributed among all frequencies, Planck's model required that the energy in the atomic vibrations of frequency f was some integer times a small, minimum, discrete energy,

Emin = hf

where h is a constant, now known as Planck's constant,

h = 6.626176 x 10-34 J s

Planck's proposal, then requires that all the energy in the atomic vibrations with frequency f can be written as

E = n h f

where n in an integer, n = 1, 2, 3, . . . No other values of the energy were allowed. The atomic oscillators could not have energy of (2.73) hf or (5/8) hf.

This idea that something -- the energy in this case -- can have only certain discrete values is called quantization. We say that the energy is quantized. This is referred to as Planck's quantum hypothesis. "Quantum" means how great or of a fixed size.

Planck did not realize how radical and far-reaching his proposals were. He viewed his strange assumptions as mathematical constructions to provide a formula that fit the experimental data. It was not until later, when Einstein used very similar ideas to explain the Photoelectric Effect in 1905, that it was realized that these assumptions described "real Physics" and were much more than mathematical constructions to provide the right formula.

Blackbody Radiation

The Photoelectric Effect
Return to Ch 28, Quantum Mechanics

(c) Doug Davis, 2002; all rights reserved