The UV–visible region of the electromagnetic spectrum extends from 200 to 800 nm.

Organic compounds with conjugated π systems strongly absorb in this region, causing electronic transitions.

Recall that 1,3-butadiene has four π molecular orbitals. The HOMO is a π-bonding orbital occupied by two electrons, and the LUMO is an empty π* antibonding orbital.

Absorption of UV light at 217 nm promotes one electron from the HOMO to the LUMO, resulting in a π → π* transition.

A plot of the absorption intensity versus wavelength gives the UV–visible absorption spectrum of 1,3-butadiene, where the absorption maxima, λ_max, corresponds to the HOMO–LUMO energy gap.

Interestingly, as the extent of conjugation increases, the HOMO–LUMO energy gap decreases, and λ_max shifts to higher wavelengths.

In highly conjugated systems, λ_max extends into the visible region, imparting color to the molecule.

For example, β-carotene, a compound found in carrots, has 11 double bonds. It absorbs strongly at 457 nm, which is the blue region of the visible spectrum, but reflects the complementary color and appears orange.