Illustrated Glossary of Organic Chemistry

Chromophore: The portion of a molecule's structure which absorbs visible light photons, causing the molecule to have color.

All-trans-retinal (molecular structure shown above) is orange. Its chromophore (shown in red) is a set of conjugated p orbitals containing five contiguous alkenes and an adjacent aldehyde. Absorption of indigo photons by retinal's chromophore causes this substance to appear orange to human eyes.