Schlieren optics and shadowgraphs
Schlieren photography shows heat shadows by revealing minute changes in the refractive index of air.
The optical technique known as schlieren photography allows us to see small changes in the index of refraction in air. A point source of light is reflected from a concave mirror and focused onto the edge of a razor blade, which is mounted in front of the camera. Tiny changes in the air’s refractive index are revealed as shadows of different intensities.
Read more about how it works.