Dyes and pigments
Coloured substances absorb some wavelengths of light and send back the rest; the colour we see is what is left over.
A dye or pigment looks coloured because its molecules absorb some wavelengths of visible light and reflect or transmit the rest. White light contains every colour from about 400 nanometres (violet) to 750 nanometres (red). Take away one band and the eye sees what remains, roughly the complementary colour: a substance that absorbs yellow light at around 580 to 595 nanometres looks blue.
Which wavelengths a molecule absorbs depends on its structure. In most dyes the colour comes from a part of the molecule called the chromophore, linked to a chain of alternating single and double bonds whose electrons are spread across many atoms. Small changes to that structure shift the colour. Plants use the same chemistry: anthocyanins give many flowers, berries and red cabbage their reds, purples and blues, and their colour changes with acidity.
For most of history dyes came from plants, insects and shellfish, and some were fabulously expensive. In 1856 an 18-year-old chemistry student, William Henry Perkin, was trying to make the malaria drug quinine from chemicals derived from coal tar. Instead he produced a purple substance, mauve, the first commercial synthetic dye, and helped launch the modern chemical industry.
Photochemistry, chemistry driven by light, can also create colour: light can break and rearrange the bonds of a colourless molecule and turn it into a coloured one, as happens when some shellfish purples develop in sunlight.
Coming up in future stories
- The Purple Dye That Cost 10,000 Snails a Gram c. 1500 BC–AD 400 · Tyre, Lebanon
- The Teenager Who Turned Coal Tar Purple 1856 · London, England
- Tulip Mania Was Caused by a Virus 1637 · Dutch Republic