Quinine is a naturally occurring substance known to have fever-reducing and anti-inflammatory properties. Originally taken from the bark of a tree known as cinchona, quinine was found to combat the effects of plasmodium falciparum, the cause of malaria.
Because of these medical properties, quinine became very popular in the 19th century. Though other medicines have replaced it for treating malaria, quinine continues to be available with a prescription for certain uses.
The applications of quinine for medical treatments led to its use as an additive in water. This combination is known as "quinine water," and in many parts of the world is referred to as "tonic water." This mixture was commonly used by travellers as a convenient safeguard against malaria.
Quinine water today does not contain large doses of the substance, however small amounts are still included. Quinine doses are restricted by law in the United States, limiting the amount to 83 parts per million in water.
Quinine is known to have a fluorescent quantum yield, which gives it a glowing effect when viewed under ultraviolet light. "Quantum yield" measures how efficiently absorbed light creates an optical effect.
A substance such as quinine becomes fluorescent when absorbed light photons release photons of another wavelength. When the absorbed photons are in the ultraviolet range and the triggered emission is in the visible spectrum, a substance will glow under black light.
Even in small amounts, quinine has these fluorescent optical properties. Mixtures containing quinine, including tonic water, emit visible photons when subjected to ultraviolet light, creating a blue-green light effect.