Organic compounds must exhibit conjugation in order to be visualized by UV light. Conjugation refers to the presence of alternating double and single bonds between carbon atoms in a molecule. This arrangement of bonds allows the electrons in the molecule to move freely, which results in the absorption of UV light. The amount of UV light absorbed by a compound is proportional to the number of conjugated double bonds in the molecule.

Compounds that do not exhibit conjugation, such as alkanes and cycloalkanes, do not absorb UV light and are therefore not visible under UV light.