Schlieren photography lets us see the interference patterns in light created by the different densities in the air that the light passes through. This video uses it to let us look at the shock waves given off by different guns.
Schlieren photography, a technique used to visualize shockwaves, is used to great effect in this image of a [YOUR GUESS HERE], captured by researchers Kim Vandiver and Harold Edgerton. (Hit the jump for the answer, as well as the full image.)
Yer a wizard, Harry! This hand looks like it is gathering the force necessary to hurl a fireball. Actually, it's just giving off regular body heat in a special type of photograph that manages to capture changes in air density.