If you have a camera and a long lens, then you’re halfway toward a Schlieren photography setup. YouTube channel Veritasium demonstrates the effect in the video above, essentially revealing gasses and airflow normally invisible to the human eye. All it takes is an optical-grade concave mirror, an LED, a camera on a tripod with a telephoto lens and a razor blade.
What the camera sees with everything aligned is actually the slight differences in the refractive index of whatever’s in front of the mirror. If you light a match in front of the mirror, light from the LED will change direction slightly differently as it passes through the warmer and cooler air around the flame.
We don’t normally perceive those differences, but this setup reveals them as lighter and darker spots to the camera. The same thing happens with, for example, butane escaping from a lighter. Light passes through it at a slightly different angle than the air around it, and the Schlieren rig captures those slight differences.
Suddenly, it’s possible to see the heat displaced when you rub your hands together, or worse, the stuff that flies everywhere when you sneeze. It’s pretty darn cool, especially when played in slow motion as in the video above.