Shadow blister effect
The shadow blister effect is a visual effect whereby adjacent shadows appear to bulge (or blister) towards one another when cast by one or more non-point light sources. It is the result of overlapping shadow penumbra on the surface and the perception by the brain of a contrast barrier formed by the merging of the penumbra. The visual effect is proportionate to the distance of the obstructions to the shadow surface (as the penumbras become larger), as well as the angular diameter of the light source(s). Penumbras can be easily demonstrated using Ray Theory.[1]
This effect is commonly visible on Earth by shadows cast by the sun. A common example is the shadows of leaves on the ground which are fuzzy and seem to bulge to one another, however any two objects held at an average human's height from the ground can produce this effect.
The appearance of the shadow blister may be more striking as the mind perceives a contrast line within the linear distribution of light within the penumbra (the shadow's blister).