Ernst Mach was an eponymous physicist who is best known for his work in the field of physics, particularly in the areas of fluid dynamics and acoustics. He is credited with developing the concept of Mach number, which is a measure of the speed of an object relative to the speed of sound. He also developed the Mach-Zehnder interferometer, which is used to measure the refractive index of a material. His work has had a lasting impact on the fields of aerodynamics and aerospace engineering.