The Mach effect, named after the Czech physicist Ernst Mach (1838 to 1916), is a sensory-psychological visual impression that causes the viewer to perceive brightness transitions with a higher contrast
than is actually present. Due to contrast enhancement, the edges between differences in brightness are perceived more clearly by the viewer.
The Mach effect is about the transition between two monotone color areas being interpreted in such a way that the dark color area is perceived as having an even darker edge and the color area with higher brightness is perceived as having a lighter edge. As a result, the borderline between two color areas is perceived more clearly.