Refractive index
Schlieren photography is used to visualise the flows of the media, which are themselves transparent (hence, their movement cannot be seen directly), but form refractive index gradients, which become visible in schlieren images either as shades of grey or even in colour. Refractive index gradients can be caused either by changes of temperature/pressure of the same fluid or by the variations of the concentration of components in mixtures and solutions. A typical application in gas dynamics is the study of shock waves in ballistics and supersonic or hypersonic vehicles. Flows caused by heating, physical absorption or chemical reactions can be visualised. Thus schlieren photography can be used in many engineering problems such as heat transfer, leak detection, study of boundary layer detachment, and characterization of optics.