Pistol being fired. Full-scale Schlieren photograph of a man firing a bullet from a pistol. The image shows circular shockwaves from the travel of the supersonic bullet and the muzzle blast, as well as the white clouds of propellant gas. Schlieren photographs are used to visualise heat and airflow around objects. Changes in the density of air, caused here by turbulence, cause light to refract (bend), and show up in the image as differences in colour and brightness. Photographed at the full-scale Schlieren system, developed by Dr Gary Settles, professor of mechanical engineering at Pennsylvania State University, USA.