their minerals were analysed by spectroscopic methods. The element helium (He) was discovered in the sun by spectroscopic method. Line Spectrum of Hydrogen When an electric discharge is passed through gaseous hydrogen, the H molecules dissociate and the energetically excited hydrogen atoms produced emit electromagnetic radiation of discrete frequencies . The hydrogen spectrum consists of several series of lines named after their discoverers.
Balmer showed in on the basis of experimental observations * The restriction of any property to discrete values is called quantization. that if spectral lines are expressed in terms of wavenumber ( ), then the visible lines of the hydrogen spectrum obey the following formula: ( . ) where n is an integer equal to or greater than (i.e., n = , , ,....) The series of lines described by this formula are called the Balmer series . The Balmer series of lines are the only lines in the hydrogen spectrum which appear in the visible region of the electromagnetic spectrum.
The Swedish spectroscopist, Johannes Rydberg, noted that all series of lines in the hydrogen spectrum could be described by the following expression : ( . ) where n = , ........ n = n + , n + ...... The value , cm – is called the Rydberg constant for hydrogen.
The first five series of lines that correspond to n = , , , , are known as Lyman, Balmer, Paschen, Bracket and Pfund series, respectively, Table . shows these series of transitions in the hydrogen spectrum. Fig. .
(page, ) shows the Lyman, Balmer and Paschen series of transitions for hydrogen atom. Of all the elements, hydrogen atom has the simplest line spectrum. Line spectrum (a) (b) Fig. .
(a) Atomic emission. The light emitted by a sample of excited hydrogen atoms (or any other element) can be passed through a prism and separated into certain discrete wavelengths. Thus an emission spectrum, which is a photographic recording of the separated wavelengths is called as line spectrum. Any sample of reasonable size contains an enormous number of atoms.
Although a single atom can