A highlighter line looks brighter than any ordinary yellow, and under an ultraviolet lamp it shines. Its ink is fluorescent: it takes in light of one colour and gives out light of another.
In fluorescence the energy to lift an electron comes from light. The molecule absorbs light, as any coloured molecule does, and an electron jumps up. Before it falls back, the molecule jostles its neighbours and loses a little of the energy as heat. Then, within billionths of a second, the electron falls the rest of the way and the molecule gives out light. The drop is smaller than the jump, so the light given out carries less energy and has a longer wavelength than the light taken in.
Tonic water
Tonic water tastes bitter because of quinine, a medicine for malaria from the bark of the cinchona tree. In the United States tonic water may hold up to 83 mg of quinine in a litre. Quinine absorbs ultraviolet light around 350 nm and gives out blue light around 450 nm, so under an ultraviolet lamp a glass of tonic water glows bright blue.
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Highlighters and white shirts
Yellow highlighter ink holds pyranine, a dye built on four benzene rings joined edge to edge. It absorbs blue light and ultraviolet, and gives out green light around 510 nm. In daylight, the ink takes the blue light that falls on it and gives much of that energy back as green, on top of the yellow and green light it reflects. It sends more yellow-green light to your eye than an ordinary yellow ink can, so it seems to glow.
| Where | Absorbs | Gives out |
|---|---|---|
| tonic water: quinine | ultraviolet | blue |
| yellow highlighter: pyranine | blue and ultraviolet | green |
| washing powder: brighteners | ultraviolet | blue |
| leaves crushed in ethanol: chlorophyll | violet, blue and red | red |
Washing powders hold optical brighteners, fluorescent molecules that stay in the cloth. They turn the ultraviolet in daylight into a little extra blue light, which hides any yellowing, so white shirts look whiter.