Motorcycle, Schlieren image. The hot engine of the motorcycle is producing a rising plume of heated air. The heated air is revealed by Schlieren photography, a way of visualizing density changes in transparent materials. The density change (here caused by heat and convection turbulence) causes light passing through the air to bend (refract). The imaging method alters the brightness or colour of this refracted light. The method is being used to study heat and air flow in places like offices, homes and kitchens. Photographed by the world's only (as of 2003) full-scale Schlieren system, developed by Gary Settles, professor of mechanical engineering at Pennsylvania State University, USA.