We all know there are plenty of invisible things floating around us, but we rarely see them. Think of the heat of a match or helium leaving a balloon. We know they're there because we can feel our hands grow warm, or see the balloon deflate, but we can't see them.
Well, it turns out that you actually can see these things — and all from the comforts of your own home.
Some of these invisible things, like the heat of the match, are called schlieren, and we've actually been observing them for a few centuries now. In fact, you've been able to see them, even if you didn't realize what you were looking at, like when you see that wavy haze coming off a hot road. The basic gist is that with the right set-up, we can see the changes of density in transparent things, i.e., heat into air.
So while we've been able to observe these in some fashion since the 1600s, it wasn't until the 1860s, when a German physicist named August Toepler set about to study shock waves, that a system was developed to capture the changes in air density by measuring changes in light and how it bends. (Physics, as the link notes, is weird in that you rarely directly measure the thing you want to measure.) Since then, the use of the schlieren imaging has been used by the likes of NASA to test aerodynamics.
The video above, from Derek Muller, shows you how to do this at home. You do need a pretty precise set-up — he has a DSLR, a small concave mirror, a tiny LED bulb and a razor blade — to capture the changes in how light bends as the air responds to released helium or a match being lit. And if you don't want to pull these items together, you can still enjoy the stunning sights that schlieren imaging creates by looking under the air's invisibility cloak by watching this explainer video.