Schlieren photography is actually a pretty commonly used system in high-speed experimental aerodynamics. A typical schlieren system will shine a collimated light source on the target (a wind tunnel test section or, above, a candle), bounce that light off a mirror, block half the light with a knife-edge at the focal point, and then record the subsequent images with a camera (high-speed or otherwise). The density of air is closely related to its index of refraction, so light that hits air of a different density will be bent more or less than a neighboring ray. This uneven bending of the light rays due to density gradients is what causes the light and dark areas on the schlieren images. Since the density of air changes drastically across a shock wave, the schlieren system is perfect for visualizing shock waves and has, in fact, been used for that purpose since 1864!