Schlieren photography made invisible shock waves in air directly observable.

Evidence

Schlieren is the German word for optical inhomogeneities in transparent material. Schlieren photography, which makes even slight inhomogeneities visible, was invented by the German physicist August Toepler in 1864. He succeeded in photographing shock waves in air created by supersonic (faster than the speed of sound) objects. A sharp pressure pulse (shock wave) wave can be created by an electrical spark; we hear this wave as a sudden “snap.” — Eric J. Heller, Why You Hear What You Hear- An…, 199

Categorization

• DEFINE: Defines schlieren and grounds it in Toepler’s 1864 invention.
• XREF: Ties to auditory perception of the spark’s snap as a sharp pressure pulse.