Chromophore

A chromophore is the part of a molecule responsible for its color. The word is derived from Ancient Greek χρῶμᾰ (chroma) 'color' and -φόρος (phoros) 'carrier of'. The color that is seen by our eyes is that of the light not absorbed by the reflecting object within a certain wavelength spectrum of visible light.

The chromophore is a region in the molecule where the energy difference between two separate molecular orbitals falls within the range of the visible spectrum.
Visible light that hits the chromophore can thus be absorbed by exciting an electron from its ground state into an excited state.
In biological molecules that serve to capture or detect light energy, the chromophore is the moiety that causes a conformational change in the molecule when hit by light.

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Source: Wikipedia · Image: Wikimedia Commons · CC BY 3.0