UV

Meaning: A photon is absorbed when its energy, E = hc/λ, matches the gap between a filled orbital (HOMO) and an empty one (LUMO). The electron is promoted, usually π → π* or n → π*. Conjugated π systems have smaller gaps, so they absorb at longer wavelengths. Isolated C=C bonds and saturated compounds generally absorb below the usual 200 nm cutoff, so they are effectively transparent.

Equation: A = εlc, where A is absorbance (unitless), ε is molar absorptivity (m²/mol in SI; usually M⁻¹cm⁻¹), l is path length (m; usually cm), and c is concentration (mol/m³; usually mol/L).

On the MCAT: Expect questions that rank compounds by λmax. More conjugation gives a longer λmax (a red shift). Conjugated dienes, enones and aromatic rings absorb, and isolated alkenes do not. Extended conjugation, as in β-carotene, can push absorption into the visible range and give color. Beer's law questions give an absorbance and ask for concentration. Biology passages use the same idea, with nucleic acids absorbing near 260 nm and proteins near 280 nm because of aromatic residues.

Common confusion: Students expect UV to identify functional groups the way IR does. IR reports bond vibrations. UV mainly reports the extent of conjugation. Also, concentration changes the absorbance but not the position of λmax.