Tonic water in UV light. Glass of tonic water and lemon seen in ultraviolet (UV) light. Tonic water contains quinine which fluoresces in UV light. Quinine is a plant alkaloid obtained from the bark of the cinchona tree. It has numerous medicinal properties, including an effective treatment against malaria. Quinine is added to soft drinks for its bitter flavour. See C004/1311 for identical image seen under normal light.