In supersonic flow, air experiences very large changes in density as it approaches, passes through and trails shock waves. As the density of air changes, so does its index of refraction. Schlieren Systems let you see shock waves as a combination of bright and dark areas. In the study of supersonic flow, shock and expansion waves are used to determine Mach number. Schlieren are optical in homogeneities in transparent materials. In most situations, however, they are virtually invisible to the naked eye.
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