Most of us don’t really understand how mirrors work, which makes for some fun reflective deception

Susana Martinez-Conde and Stephen L. Macknik in Scientific American:

We are surrounded by mirrors all day, every day—when we drive, brush our teeth, check our hair while heading out the door. Yet for all their ubiquity, mirrors remain somewhat mysterious. In folktales and fiction at least, they can be conduits to spiritual, magical or supernatural realms: mirrors can out the soulless vampires in our midst. They can summon the legendary hook-handed murderer known as Candyman. And the Mirror of Erised—of Harry Potter fame—holds the remarkable power to lay bare its viewer's deepest desire.

Our enchantment with mirrors may stem in part from the fact that they often defy expectations. Not only do we find the right-left reversal of reflecting surfaces discomfiting, but many of our hard-won intuitions about how mirrors work are dead wrong. Psychologist Marco Bertamini of the University of Liverpool in England and his colleagues have identified three false beliefs we typically have about mirrors: First, people usually predict that they will see themselves in a mirror before they arrive in front of it. In other words, they overestimate what is visible in a mirror. This miscalculation is called the “early error.” Second, most people assume that their projection on a mirror (the outline they could trace with a pen on its surface) is the same size as their body. In reality, that projection, as they see it, is half the physical size of their body. Third, people tend to think that the mirror projection of their own image will shrink with distance, so they will see their full body in a small mirror if they move far enough away from it. But in fact, distance does not affect the size of a body's projection. Moreover, some research indicates that people see objects in a mirror as somehow less real than nonreflected ones. The illusions we present here all take advantage of how little we grasp about the looking glass.

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