Did you know that tonic water fluoresces under UV light? It contains quinine, which has a high fluorescent quantum yield. “Quantum yield” measures how efficiently absorbed light creates an optical effect. In fact, it has such a high yield it will even glow under regular sunlight. 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. Throw a science themed costume party this Halloween and serve your guests radioactive drinks!