Matter absorbs energy and increases its internal energy. Depending upon the energy magnitude, matter utilizes the absorbed energy, for electronic transitions, atomic vibrations, rotation, ionization, bond dissociation etc. This means that matter goes to another state of higher internal energy by the absorption of the radiation. Earlier, it was believed, that matter could absorb any energy and go to a higher energy state.
In Newtonian times of 18th century, electromagnetic radiations, being massless was, considered to be fully wave in nature and of continuous in energy distribution. Wave nature of electromagnetic radiations, could explain some properties, but failed in the case of black body radiations, photoelectric effect, etc.
Max Planks introduced a concept of particle nature to the electromagnetic radiations. and explained the Black body radiation and the absence of ‘ultraviolet catastrophe’.
Max Planks suggested that the all-electromagnetic radiations are made of small units of energy, called ‘quanta’. Energy of the quanta is proportional to the frequency of the radiation frequency (E=hν). He proposed that matter also would either absorb or emit radiations of specific energy in the form of discreet particles of quanta and not all energies present in the continuous spectrum.
Energy Level Diagram
At any temperature above 0 K, matter, possess internal energy related to the temperature. This is the initial state of the matter. In the presence of an energy source (heating or radiations), matter absorbs energy, and goes to a higher energy state having more internal energy, which is equivalent to the absorbed energy. A pictorial representation of the magnitude of the energy, the same matter may have before (Initial state) and after (upper state) the absorption of energy is the energy level diagram.
On removal of energy source, matter releases energy, such that the emitted energy is equal to the amount absorbed. So, the matter in the final state returns to the initial state of original energy. So, energy level diagram shows the possible energy the matter may have during either absorption or emission.