Optical illusion is a visually perceived image that differs from reality. As we look down a long, straight road, we see that it seems to grow narrower in the distance. Trees and telegraph poles along the road appear to grow smaller as they stretch away toward the horizon. A white house looks larger than the same house painted a dark color. A man wearing a suit with up-and-down stripes looks thinner than he would if the stripes went crosswise. We call appearances of this kind optical illusions because we know that in such cases things are not the way they appear.
Optical illusions of the kind described above are called “normal” illusions. Every person with normal eyesight experiences them. But an optical illusion does not occur every time we are deceived by what we see. Often we make mistakes in interpreting the impressions our eye receives. For example, many persons will read the sentence “he walked though the busy street,” and never notice that the third word has no “r” in it. They expect to see “through” and therefore they do see “through.” Such mistaken impressions are not optical illusions.
