Try This Experiment...
Get two similar colorless, clean drinking glasses or jars. Fill one with ordinary tap water and the other with tonic water. Set both containers on a piece of dark cloth in direct sunlight.
Look down through both liquids. In the tonic water you will see a faint blue glow against the dark background. In comparison, ordinary clean tap water does not glow.
What Is Quinine?
That blue glow comes from quinine, the substance that gives tonic water its bitter taste, shining by the process of fluorescence. A material is said to be fluorescent if it emits light of one color whenever it is exposed to light of some other color from another source.
In this particular case you are exposing quinine to ultraviolet light, the invisible component of sunlight that produces sun tans and sunburns. The structure of the quinine molecule enables it to take in energy in the form of invisible ultraviolet light and immediately radiate some of that same energy in the form of visible blue light.
Try A Black Light!
Another case of fluorescence we know from everyday experience is the glow of a fluorescent tube. In that case, ultraviolet light emitted by energized mercury vapor inside the lamp is converted into visible light by a special material applied to the inside of the glass tube.
Special fluorescent tubes sometimes called black lights emit mostly ultraviolet light and very little visible light. Hold a glass of tonic water near a black light, if you have access to one, and you'll see a much brighter version of the same blue glow that appears when you expose tonic water to ordinary sunlight.