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Pistol being fired, schlieren image. .22 calibre pistol and its bullet just after the trigger is pulled. Propellent gas can be seen in the wake of the bullet and there is a spherical shock wave centred on the end of the barrel. Convection currents are visible from the hand of the person firing. Schlieren photography provides a way of visualising density changes in transparent fluids. Light passing through a density change is refracted (bent). The imaging technique alters the brightness and colour of this refracted light to highlight these areas. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR. GARY SETTLES)