ABSTRACT
In the late 19th century, scientists started to notice that there
were phenomena that could not be explained by the classical the-
ory of mechanics. One of them was the spectrum of absorption
and emission by atoms. According to classical mechanics, the spec-
trum should be continuous, i.e., the emitted or absorbed light could
have any frequency (wavelength). The experimental results, how-
ever, were surprising. The atoms emitted and absorbed light with
discrete values of frequency only.
