1 Black Light (easily purchased at Walmart or online)

1 Bottle of Tonic Water OR 1 Highlighter Marker

A Dark Room

(If you chose a highlighter over the tonic water you will need a small glass with a small amount of water. Make sure that it is a clear glass.)


1. If you chose to use a highlighter, ask your parent or guardian to carefully break the pen open. Remove the felt from the marker and place it into the glass holding the water. Let the highlighter soak in the water for a few minutes. You can choose to leave this highlighter felt in the water or you can then remove it.

2. Take the glass of water into a dark room.

3. Turn on the black light and shine it near the glass of water. The water should be illuminated.


The ultraviolet light that is being emitted from your black light excited the phosphors within the water. The dye in the highlighter and tonic water have phosphors in them. This is what is causing your water to glow under the UV light. The UV light is light that we cannot normally see. The phosphors pick the light up and allow you to see the ultraviolet light. This is why the water is glowing! Did you know that a black light is also used in forensic science? Ultraviolet light helps catch bad guys! It’s pretty cool!

There are quite a few different types of luminescence. Fluorescence is used in this particular experiment and will only glow when the black light is shining on it. Phosphorescence has a lasting glow even when the black light is not shining on it. Chemiluminescence is used in such things as glow sticks. Bioluminescence is when living organisms glow! You can use this experiment in a multitude of different demonstrations.