Sometimes our eyes play tricks on us. Even when we know what is actually happening, our visual system won’t let us see the reality. Michael Bach has an extensive collection of 141 optical illusions, along with explanations of what’s tripping up:
Optical illusion sounds derogative, as if exposing a malfunction of the visual system. Rather, I view these phenomena as highlighting particular good adaptations of our visual system to experience with standard viewing situations. These experiences are based on normal visual conditions, and thus under unusual contexts can lead to inappropriate interpretations of a visual scene (=”Bayesian interpretation of perception”).