According to the third postulate of Bohr’s model, when an atom makes a transition from the higher energy state with quantum number \(n_i\) to the lower energy state with quantum number \(n_f\left(n_f<n_i\right)\), the difference of energy is carried away by a photon of frequency \(\nu_{i f}\) such that
\(
h \nu_{i f}=E_{n i}-E_{n f} \dots(12.11)
\)
Since both \(n_f\) and \(n_i\) are integers, this immediately shows that in transitions between different atomic levels, light is radiated in various discrete frequencies.
The various lines in the atomic spectra are produced when electrons jump from higher energy state to a lower energy state and photons are emitted. These spectral lines are called emission lines. But when an atom absorbs a photon that has precisely the same energy needed by the electron in a lower energy state to make transitions to a higher energy state, the process is called absorption. Thus if photons with a continuous range of frequencies pass through a rarefied gas and then are analysed with a spectrometer, a series of dark spectral absorption lines appear in the continuous spectrum. The dark lines indicate the frequencies that have been absorbed by the atoms of the gas.
The explanation of the hydrogen atom spectrum provided by Bohr’s model was a brilliant achievement, which greatly stimulated progress towards the modern quantum theory. In 1922, Bohr was awarded Nobel Prize in Physics.
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