A colour is a physical object as soon as we consider its dependence, for instance, upon its luminous source, upon other colours, upon temperatures, upon spaces, and so forth.

Up to the age of 14 Mach received his education at home from his parents. Using schlieren photography he and his son Ludwig were able to photograph the shadows of the invisible shock waves. During that period Mach continued his work in psycho-physics and in sensory perception.

Ernst Waldfried Josef Wenzel Mach (German: [ˈɛɐnst max]; February 18 1838 – February 19 1916) was an Austrian physicist and philosopher noted for his contributions to physics such as the Mach number and the study of shock waves.