White's illusion
The belongingness theory has been given as an explanation of White's illusion. Belongingness means that we perceive things depending on its surroundings. According to belongingness theory, the lightness of rectangle A is influenced by the white bars that surround it. Similarly, the rectangle B on the right side is surrounded by the dark bars. So, the lightness of rectangle B is affected by the dark background. As a result, area A which rests on the white background appears darker than area B which rests on the dark background.