Editing Ernst Mach (section)

==Physics==
Most of Mach's initial studies in experimental physics concentrated on the [[Interference (wave propagation)|interference]], [[diffraction]], [[Polarization (waves)|polarization]] and [[refraction]] of light in different media under external influences. From there followed explorations in [[supersonic]] fluid mechanics. Mach and physicist-photographer [[Peter Salcher]] presented their paper on this subject{{sfn|Mach|Salcher|1887|pp=764–780}} in 1887; it correctly describes the sound effects observed during the supersonic motion of a [[projectile]]. They deduced and experimentally confirmed the existence of a [[shock wave]] of conical shape, with the projectile at the apex.{{sfn|Scott|2003}} The ratio of the speed of a fluid to the local [[speed of sound]] ''v''<sub>p</sub>/''v''<sub>s</sub> is called the [[Mach number]] after him. It is a critical parameter in the description of high-speed fluid movement in [[aerodynamics]] and [[hydrodynamics]]. Mach also contributed to [[physical cosmology|cosmology]] the hypothesis known as [[Mach's principle]].<ref name="Reichenbach"/>