世界上为什么会有影子？（英文版）

http://en.wikipedia.org/wiki/Shadow
A shadow is a region of darkness where light is blocked. It occupies all of the space behind an opaque object with light in front of it. The cross section of a shadow is a two-dimensional silhouette, or reverse projection of the object blocking the light. It begins as the light moving through but is blocked depending on the density.