The video below looks just like magic, how can something just change by itself right in front of our eyes? Well, with some understanding of waves we can explain this phenomenon. The big idea, or sole reason to this so called magic is refraction. When you look at the glass the light rays bend around the center of the glass, causing the light rays to intercept each other between the glass and the arrow, or piece of paper, creating a focal point, the light that was on the right side is now on the left and vice versa, this causes the arrow to appear to be inverted, which makes it appear to be backwards. Anytime light light goes through one medium into another, it refracts. In conclusion this is in fact not magic, but can be straight forwardly explained using wave physics.