The arrow seen here appears to reverse due to refracted light. The light must travel through the air, glass, water, back of the glass and again through the air before landing on the arrow. When light passes from one material into another, it is refracted. The arrow appears to reverse itself because the glass of water acts as a magnifying glass. When light shines through a magnifying glass, it bends toward the center. The point in which the light appears to join together is the focal point. Past the focal point, the image appears to reverse, because the bent light rays pass each other. The light on the right side is now on the left and vice versa, thus the arrow appears reversed. Via Science Dump.