The photographic technique known as Schlieren photography gets its name from the German word "Schliere", meaning "streak". It was first developed to visualize the airflow around objects such as the wings of airplanes.
How does Schlieren photography work? It produces well contrasted-pictures based on the difference in gas or liquid density. For example, when a gas is heated it expands, therefore, its density drops; as the temperature cools, it contracts and its density increases. When collimated (in parallel) light rays pass through fluidic materials with different densities, they refract and produce shadow-like area in the image.
In the classic setup, the parallel rays are focused onto a single point by literally using the edge of a knife. Many of the current optical schlieren methods replace knife-edge with a colored target, which produces schlieren shadows with a rainbow hue that adds extra features to visualization.
Source: Veritasium via Youtube