When double bonds are separated by just one single bond they are said to be conjugated . A prime example of a conjugated system is the benzene ring.
The part of the molecule which absorbs radiation mainly within the ultraviolet/visible region of the electromagnetic spectrum is known as a chromophore. A chromophore is an unsaturated group of atoms within an organic compound. This maybe a triple bond, double bond or a delocalised system. If a molecule exhibits conjugation then it is able to absorb electromagnetic radiation. Benzene absorbs electromagnetic radiation at approximately 200 nm. This is outside of the visible region so it appears colourless to the human eye. The visible region of the elecromagnetic spectrum is between 400-700 nm. If a molecule absorbs radiation between 400-700 nm it will exhibit different colours depending on the wavelength.