This section is the analytical companion to the two above: UV-Vis spectroscopy is how conjugation is detected and measured. The core idea is that molecules absorb ultraviolet or visible light when the energy of a photon exactly matches the gap between a filled and an empty molecular orbital, promoting an electron — most commonly a π → π* transition (and, for lone-pair-bearing groups, n → π*).

The key relationship to conjugation:

More conjugation → smaller HOMO–LUMO energy gap → absorption at longer wavelength (lower energy). Each additional conjugated double bond extends the pi system, raises the HOMO and lowers the LUMO, and shifts absorption to longer λ. A shift to longer wavelength is a bathochromic ("red") shift.
An isolated alkene absorbs in the far UV (~170 nm); a conjugated diene absorbs higher (~217 nm); extended polyenes climb further still. When conjugation becomes long enough that absorption reaches into the visible region (~400–700 nm), the compound is colored — this is why β-carotene, retinal, and many dyes and pigments are colored, and it is the tie-back to biological chromophores.
A chromophore is the conjugated portion of a molecule responsible for absorption; auxochromes (e.g., –OH, –NH₂) attached to it shift the absorption further.

The quantitative tool is the Beer-Lambert law: A = εbc, where A is absorbance, ε is the molar absorptivity (an intrinsic constant of the compound), b is path length, and c is concentration. Because absorbance is linearly proportional to concentration, UV-Vis is a workhorse for quantifying concentration — the practical link to the spectrophotometry students will use constantly in biochemistry and clinical labs (e.g., reading nucleic acid or protein concentrations).

"Catch the Color"