UV-Visible Spectroscopy studies the electronic transitions within a molecule.

• Detection: It is extremely useful for identifying chromophores - groups that absorb light. Specifically, it detects conjugated $\pi$ systems (like in polyenes or aromatic rings) and carbonyl groups.
• Requirement: Saturated compounds (like alkanes) only show $\sigma \to {\sigma }^{\ast }$ transitions, which occur in the vacuum-UV region ($<200\text{ nm}$) and are typically not measurable with standard instruments. Conjugated systems lower the energy required for excitation, shifting the absorption into the measurable UV or visible range.