Fluorescence

I'd been demonstrating fluorescence using a UV flashlight and a Walgreens eye drops bottle, when it occured to me that the same phenomenon has to occur at other frequencies too; ones we can't see. How about infra-red?

Yep. Visible light can be absorbed to spur IR emission (so can UV, of course), and if you filter out the visible light you can see the IR fluorescence. It turns out to be useful in identifying certain pigments like Egyptian Blue.

And, perhaps not surprisingly, one common chemical that absorbs visible (blue and red) and emits infrared is chlorophyll.

If you want to get into the infrared game, you can modify your camera.