[mod note: posts re-located from here: https://www.physicsforums.com/threads/how-does-light-slide-sideways.804112 ] Some people don't like the term "optical illusion" in a science discussion because it implies that what you are seeing isn't real. I disagree. An optical illusion is an image that is just interpreted incorrectly because of a misunderstanding or incomplete (or too much irrelevant) information. A great many optical illusions involve a form of relativity: objects that look different from different locations, camera angles, states of motion, etc. The trick is simply making you draw a conclusion about how it looks in one frame of reference based on information provided in another.