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Despite its extraordinary success in explaining the majority of physical phenomena studied by scientists up to that time, at the start of the 20th century a small number of observations still stubornly resisted explanation by classical dynamics.

A number of these issues are adverted to in Understanding Physics, particularly in the chapter on thermal physics (the blackbody radiation problem in Section 11.7, the internal energy of gases at low temperature at the end of Section 12.2 and the heat capacity of solids at low temperatures in Section 12.8). Other phenomena defying explanation at that time included the photoelectric effect, the high frequency cut-off in the X-ray continuum and the characteristic lines in the visible spectra of hot gases.

The first seven sections of Chapter 14 outline the experimental basis and related issues that ultimately gave rise to the development of quantum mechanics. It is difficult to underestimate the revolutionary nature of the ideas involved and their impact, not only on physics, but also on chemistry, engineering and many other fields.

Before starting this chapter you might like to remind yourself of the early part of the history of physics in the 20th century as described in the American Physical Society's A Century of Physics website  (recall Section 1.1 of Understanding Physics).