The solid black curve represents the amount of light being reflected from the figure at the top. The red curve represents the brightnesses of this figure as it is usually perceived.
To the left of the point where the figure just starts to get lighter people usually see a dark bar that is slightly darker that the area to the left of it.
At the point where the brightness just stops increasing, people usually perceive a bright bar.
This phenomenon was discovered by the famous physicist, Ernst Mach
and it is in his honor that these dark and bright bars are called Mach Bands. These Mach
Bands can be explained by center-surround receptive field
interactions which is discussed elsewhere in this book.