Shine a blacklight on your slime balls to make them glow. What's going on? This project uses a "black light" to make a glowing bouncy ball. Black lights look a little bit purple, but it's the light they produce that you can't see that's really interesting. Black lights produce ultraviolet (UV) light. Our eyes can't perceive UV light, but our skin can feel it; UV light is the reason we get sunburns. Some substances, like fluorescent paint, will glow under UV light. The paint absorbs the energy in the UV light, and then emits it as visible light. Groovy!