Evidence for Energy Quantization in Atoms

The sources of blackbody radiation, according to classical physics, are oscillating electrical charges in the surfaces of these objects that have been accelerated by ordinary thermal motion. Each motion persists for a certain period, producing radiation whose frequency is inversely related to that period. A number of scientists used different methods to calculate the radiation intensity curves using this simplified model and arrived at the following result  

FIGURE 4.6 The dependence of the intensity of blackbody radiation on wavelength for two temperatures 5000 K (red curve) and 7000 K (blue curve). The sun has a blackbody temperature near 5780 K, and its light-intensity curve lies between the two shown. The classical theory (dashed curves) disagrees with observation at shorter wavelengths. 

A charged particle of mass m bound to a solid surface by a spring Is a model for the oscillatory motions of the surface atoms of a black body. The particle Is shown at its rest position Xq, at a position closer to the surface (x Xo), and at one further from the surface (x Xq). 

The potential energy curve for an oscillator is a consequence of the restoring force that always drives the oscillator toward its equilibrium position. 

Blackbody radiation was explained by Max Planck in 1901, but only by overthrowing the very foundations of classical mechanics. Planck reasoned that the very high-frequency oscillators must not be excited by the thermal energy of the hot body to the same degree as the lower frequency oscillators. This was a challenge to explain because classical mechanics allows an oscillator to have any energy. Planck s argument involved two steps, which are explained as follows. 

---