Why is the Sky blue?

Floor 2 | Light and Vision
Why on earth is the sky blue and not black as night? After all, we are looking directly into the almost black universe. A slide projector shining through a tube filled with semi-transparent liquid reveals it: the originally white light is scattered by tiny parts as it passes through the tube (or sky). At first, only the blue parts of the light (day) are scattered, but as the path becomes longer, the red parts of the light (evening) are also scattered.