Although I am 41, I still found this interesting: http://www.engineeringinteract.org/resources/alienattack/flash/concepts/shadows.htm There are degrees of opaqueness. My hand causes a shadow when light shines on it. But if I hold my hand up to the light source it changes colour so some light is emitted through it. Shadows are darker when the light source is very bright. As the sun sets, the row of houses on the western horizon becomes almost black: a silhouette.