Shadow blister effect

The shadow blister effect is a visual phenomenon in which a shadow bulges (or blisters) as it approaches another.
The effect takes place when two objects are located at varying distances between a non-point light source and a backdrop upon which their shadows are cast. As the two objects move transversely such that their shadows approach each other, the object nearest the light source will begin blocking light from reaching the inside of the other object's penumbra, thereby expanding its umbra. This expansion of the further object's umbra will continue until the umbras of both objects meet.
This effect can be demonstrated and understood using ray theory.
Common misconceptions.
The shadow blister effect is commonly misconceived to be an illusion caused by the combining of the two object's penumbras, aided by factors such as diffraction, nonlinear response, and the eye's inability to differentiate between varying contrasts.