Pakistan study notes for Matric, Intermediate, B.A, B.COM BSC, M.A, BCS. Free Pakistani Urdu educational school, colleges and University notes.



Historical background:

It is well known that when a gas is heated in a discharge tube at low pressure, it emits radiation. Balmer in 1885, studied the spectrum of hydrogen gas. He found a series of lines in visible region (i.e, having A between 4000 to 8000 A°). They were called Balmer series. He proposed an empirical formula to find wave number u of each line.


Brackett found another series infar-infrared region.

Pfund also found another series infar-infrared region.

General expressions: A general expression can give wave number of each line of each series. It is:




Bohr’s theory success lies in the fact that it provided logical proof and reason for the formation of hydrogen spectrum. According to Bohr’s theory, “At ordinary temperature, the electron in hydrogen atom resides in lowest energy level i.e, first orbit or ground state. When electrically it is heated at low pressure in a discharge tube, the electrons of different hydrogen atoms absorb different amount of energy and jump to an appropriate high energy level. They are now said to be in excited state.

All the atoms in excited state, due to their high energy, are unstable. So now the electrons jump back to the original first orbit directly or to some other level of lower energy. In doing so they emit energy equal to the difference of energies of two levels. These energetic waves are separated by a prism according to their wavelength and thus hydrogen spectrum is obtained.

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