colour

The Chemistry of Colour
Coloured chemicals absorb electromagnetic waves in the visible part of the spectrum. The absorbed energy
causes changes in the energy of the molecules’ electrons. The electrons change from a ‘ground
state’ to an ‘excited state’.
Most transitions are not caused by visible light. Many asorb ultra-violet
radiation. Chemicals which absorb UV radiation are colourless (unless they fluoresce). The
energy changes when molecules of a coloured compound and of a colourless compound are illustrated below:
below:

Remember that the apparent colour is caused by absorbing photons of a complementary colour. A
blue compound is blue because it absorbs yellow light.


The hexagon has the three primary colours of light (RGB), and their complementary colours (CYM).