system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the light that dentists use to harden fillings, and how does it harden it so quickly?
gpt
That light is an LED curing light. The fillings contain a certain chemical that reacts to light emitted at very specific wavelengths on the blue part of the spectrum. When exposed to that light, it triggers a reaction that causes thefilling to cure pretty much instantly

The reason they use this sort of light in particular is because other methods of curing would be unreliable. Natural light or air or moisture curing would all harden immediately. 

But using these lights and the chemicals that react to themmeans there's no chance of accidentally exposing it to the curing method until the exact moment they wanted to.