Moving shadows are a strong cue to depth change. In this demonstration, the yellow square remains fixed in position on the screen, but when the grey area representing its shadow drifts downward to the right it appears to move forward away from the background. The cue overrides contradictory information regarding the unchanging size of the square. The strength of the effect does depend on the properties of the shadow region. For example, shadows with a penumbra, and which fall below the object, work best.
According to Kersten et al. (1997) the existence of the illusion shows that our perception of the spatial layout of scenes relies on the assumption that the light source responsible for the shadow is stationary, so that any motion of the shadow is attributed to the object casting it.