Absorption spectrum and Emission spectrum

These two phenomena are the evidences for the resonant columns in atoms. When a cooled gas is placed in the path of a continuous spectrum of light, dark absorption lines will appear in the resulting spectrum. Similarly, if we observe the gas in oblique angle, emission lines will be visible against a dark background. When a photon falls on an electron and the resonant frequency of the shell (electron shell or transitory shell) in which the electron presently exist matches with the frequency of the photon, the photon will be efficiently absorbed by the atom. In the resultant spectrum, the absorbed photons will not be present. This is the reason for the absorption line.
   When a shell's electron absorbs a photon, the shell oscillates with the frequency of the photon. This oscillation of the shell causes its electron to create a photon in the same frequency of the incident photon, i.e. emission line is developed by reflected photons.

Zeeman Effect, Paschen-Back effect & Stark effect

   The electron configuration in a multi electron atom is determined by three factors. a) Attraction from the nucleus b) repulsion between electrons and c) buoyant force exerted by space matter (see structure of atom). When an atom is situated in a strong- electric field or magnetic field, its electron configuration is shifted from its normal state i.e. because of the influence of the external field, some electrons in the atom (mostly outer electrons) are shifted from their original shells. When such an atom is excited, the atom can emit different spectrum of radiations compared to its original spectrum lines. This effect is a direct indication for the strength of the field i.e. when a weak field affects outer electrons only, a strong field can influence both the inner and outer electrons alike. Lighter elements can demonstrate the effect in a weaker field comparatively than heavy elements because the binding force on the electrons in a lighter element is much less than that of heavy elements.