Spectroscopic transitions occur when electrons move between discrete quantum energy levels, absorbing or emitting photons with energy matching the level difference.

In quantum systems such as atoms or molecules, electrons occupy quantized energy states rather than a continuum. Transitions between these states require precise photon energies equal to the energy gap, leading to absorption or emission lines characteristic of the system's structure. This principle underpins spectroscopy’s ability to reveal chemical composition and electronic structure with high specificity.