Which of the internal squares is bigger? Black or white? Irradiation phenomenon lies in the fact that light objects on the dark background seem bigger compared to their real size and as if occupy some part of dark background. When we view light surface against dark background, the borders of this surface as if widen due to imperfection of the lens, and this surface seems to us bigger, than its genuine geometric size. Due to colors brightness the white square seems much bigger compared to the black square on the white background. It is interesting to note that being aware of this feature of black color to make the size smaller, duelists of the 19th century preferred shooting in black suits expecting that the opponent would miss at shooting.