Last years of the nineteenth century. Experiments are being performed on gases by passing electric discharge through them at very low pressure. Little did the experimentalists knew they were on the threshold of a new order in the world of physics. Little they must have known that the results of their experiments would lead to profound changes in the understanding of nature and its laws by scientists all over the world.
The results obtained were perplexing. For each element observed, a set of discrete ( having discrete frequency or wavelength ) electromagnetic waves ( called the emission spectrum or atomic spectrum ) was obtained as opposed to a continuous spectrum. The set of electromagnetic waves was different for each element taken. It was as if every element has a unique signtaure in the form of its atomic spectrum.
The reason why at all electromagnetic waves are obtained is that the energy provided to the atoms from the discharge excites them and as they return back to their ground state ( lowest energy state ), they discard the absorbed energy in the form of electromagnetic waves. But the question was - Why only a discrete set of frequencies? Why not a continuous spectrum? The answer to this question was to lead to one of the greatest acheivements of physics in explaining the nature of the physical world - the quantum theory.
That the frequencies of the radiations were discrete led to the idea of quantization of energy of the electron in the atom.