Here's why the metalloid germanium is used for infrared camera lenses

In the visible light spectrum, standard glass is almost completely transparent, and the metalloid germanium is almost completely opaque. But in infrared light, the opposite is true.

Here's another demo:

ProTip: if you find yourself hiding from authorities looking for you with an IR camera, your best bet is to find a large cardboard box and make sure you're not too close to the surface. Make sure it's well-vented so the heat doesn't build up inside!

Image: YouTube / Cody'sBLab