Have a bottle with tonic water ready.
In a moment, I am going to turn on some special lights that contain blue, violet and ultraviolet light. We will see the blue-violet but we cannot see ultraviolet; we may detect the ultraviolet by looking at certain objects in this room. Besides hydrogen, there are many other natural detectors of ultraviolet light. Here I have a bottle with “tonic” water, the kind that you buy in a store to drink.
Display the bottle.
When I turn on the ultraviolet light, the chemicals in water will glow in a special way. This phenomenon is called “fluorescence.” Certain chemicals in the water absorb the ultraviolet and reradiate it as visible light, just like the nebulae in the pictures we just saw.
Turn on the UV lights. Some audience member’s clothing will fluoresce because of dyes or chemicals used in laundry detergents to “brighten” the clothes. The Sun’s ultraviolet rays cause the “brighter-whites-look” outside. Sometimes even teeth will fluoresce if a student gives a nice toothy grin. Have they been using fluoride toothpaste?