Minecraft wiki entry for Opacity, associated text: The primary purpose of opacity is to tell the game engine if it needs to draw behind the block; an opaque block completely obscures the view behind it, while a transparent block does not. Thus, transparency applies not only to solid blocks like ice and glass, but also to blocks like cacti, stairs, chests, and single slabs, which are not considered full blocks. Opacity also affects how light propagates through the world. Opaque blocks completely prevent light from traveling through them, while transparent blocks generally diminish the light by one light level per block (these value can be overridden however, and there are several exceptions). By this definition, transparency does not imply that a block is actually see-through, which is a quality of the item texture specified by the game or resource pack.