White's illusion is an optical illusion illustrating the fact
that the same target luminance can elicit different perceptions of brightness in
different contexts.
Note, that although the gray rectangles are all of equal luminance, the ones
sharing a longer border with the lighter context appear brighter than the ones
sharing their longest sides with the darker context.
Television screens depend on this visual illusion. Pixels that are not
illuminated are seen as black, when they are really the same dim gray seen when
the television is turned off.