Light emission occurs as a result of interaction between light and matter. Let us assume a two-level atom with levels 1 (ground state) and 2 (excited state). The energies of the ground and excited states are E 1 and E 2 , respectively, and the energy difference is therefore given by the difference ÎE 12 =E 2 âE 1 . When light with photons of energy equal to this difference is absorbed by the atomic or molecular system, an electron will be excited from level 1 to level 2. The energy of the photons is given by ÎE 12 =E 12 =hÎ½ 12 (h is Planck's constant and 1â2 is the frequency of light resulting from the level 1â2 transition). This energy is absorbed by an atom or a molecule that has energy levels, separated by ÎE, where these energy levels are the ground and the excited states. This kind of âquantum jumpâ of an electron between two states occurs in atomic systems between electronic levels; it can be extended to molecular systems, where vibrational and translational energy levels participate in the quantum jump. The interaction between light and matter involves the transition of electrons between different states. This interaction results in the absorption of photons (stimulated absorption) as well as spontaneous and stimulated emission. These processes can be described using Einstein's A and B coefficients, as will be described in the next paragraphs.
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