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Why does tonic water glow blue under ultraviolet light?
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photochemical reaction
Chain of fluorescent tunicates.
Tonic water glows blue under ultraviolet (UV) light due to the presence of quinine, an alkaloid added to the beverage to provide its characteristic bitter taste.12
This phenomenon is a process known as fluorescence. When the quinine molecules in the tonic water are exposed to UV light, they absorb the energy from the radiation, which promotes the molecules from their ground state to a higher-energy excited state. Because the excited state is unstable and short-lived, the molecules quickly release that absorbed energy by re-emitting it as light. This re-emitted light is at a lower energy level than the original UV light, which manifests to the human eye as a visible blue glow.1