Schlieren!

Forgive me while I post a Wikipedia article that I thought was cool...

Schlieren (from German; singular "schliere") are optical inhomogeneities in transparent material not visible to the human eye. Schlieren physics developed out of the need to produce high-quality lenses void of these inhomogeneities. These inhomogeneities are localized differences in optical path length that cause light deviation. This light deviation is converted to shadow in a schlieren system.