The idea that matter consists of small, indivisible particles can be traced to the Greek philosophers Leucippus and Democritus in the 5th century bce. They called these particles atoms, Greek for “not divisible.” Atomism was not widely accepted, due in no small part to the complete lack of evidence, but atomic ideas never died.
Things began to change in the early years of the 19th century. The English chemist John Dalton argued that chemical reactions could be understood if each chemical element consisted of identical atoms. The evidence for atoms grew stronger as thermodynamics and the kinetic theory of gases developed in the mid-19th century. Slight deviations from the ideal-gas law at high pressures, which could be understood if the atoms were beginning to come into close proximity to one another, led to a rough estimate of atomic sizes. By 1890, it was widely accepted that atoms exist and have diameters of approximately .
Other scientists of the 19th century were trying to understand what light is. Newton, as we have seen, favored a corpuscular theory whereby small particles of light travel in straight lines. However, the situation changed when, in 1801, Thomas Young demonstrated the interference of light with his celebrated double-slit experiment. But if light is a wave, what is waving? Work by Maxwell and others led to the realization that light is an electromagnetic wave.
This was the situation at the end of the 19th century, when a series of discoveries began to reveal that the theories of Newton and Maxwell were not sufficient to explain the properties of atoms. New theories of matter and light at the atomic level, collectively called modern physics, arose in the early decades of the 20th century.
A difficulty, however, is that we cannot directly see, feel, or manipulate atoms. To know what the theories of modern physics are attempting to explain, and whether they are successful, we must start with experimental evidence about the behavior of atoms and light. That is the primary purpose of this chapter and the next.