"It is our illusions that create the world", says Didier Cauwelaert. That's so true! The big blue sky appears flat; the sun appears to move across the sky, and at times, flat surfaces appear to have depth. These are optical illusions, wonders created by the optical mechanism of our eyes. Optical illusions are attributed to either visual perceptions or to our intellectual interpretations. A study shows that our past experiences, desires, and influences are also responsible for making optical illusions work. Look at the picture alongside. Could you make out which one's real and which one's the shadow or reflection? Well, any guess could be right; it's an optical illusion! How Optical Illusions Work Illusions often result from a mistaken judgment or erroneous vision. An error in estimating the distance to an object is due to mistaken judgment. A bright object often appears larger than a dark object of the same dimensions. When objects of contrasting colors are placed in close vicinity, a certain color may be mistaken as another. The human brain tends to group things in four types; namely, similarity, continuity, closure, and proximity. We group things of similar color to picture a particular shape. Our brain is always in search of patterns. It tries to find some kind of continuity in whatever our eyes see. Closure occurs because our brain tries to construct views of familiar objects from whatever we look at. For example, if short lines spaced from each other are arranged circularly, our brain perceives it to be a circle. In the fourth type of grouping, which is proximity, the brain groups the objects that are laid closely together. Mirage is a very well-known example in which light deceives our eyes. In this case, light rays coming from an object bend and make the object appear to be at a location it is actually not. There is a thin line of difference between an optical illusion and a hallucination. When the object appears to exist in spite of its absence, the phenomenon is known as hallucination. Lipps in his principle of mechanical aesthetic unity says that we give the form of a living being to every space and unconsciously consider some mechanical forces acting. But Wundt suggests that laws dealing with formation of the retinal image and the movement of the eye are accountable to optical illusions. Objects reflect the light that falls on them and the eye forms inverted images of the objects on the retina. The brain interprets the image and we perceive the object to be what it really is. In case our perception does not match with the physical dimensions of the object, it is surely an optical illusion. Shades of colors give depth to flat surfaces. Certain geometrical structures of objects have an illusive effect on the observer. The effort involved in raising the eyes in greater than that in turning them in a horizontal plane. It is interesting to note that due to the difference in the effort involved, our eyes perceive vertical distances to be greater than the horizontal ones of equal magnitudes. Angular illusions are attributed to involuntary movements of the eye. Lenses possess defects like spherical and chromatic aberrations. An eye has abnormalities like astigmatism and eccentricity. They all lead to optical illusions. Due to differences in the wavelengths of different colors, there are differences in their angles of reflection and refraction. Chromatic aberrations arise out of these differences. They lead to illusions like the 'advancing' and 'retiring' colors. When the eyelids are almost closed, their movements make objects appear to be moving. Judgments about direction and distance are not accurate with one eye closed. Distance and area appear to vary depending on the concentration of objects placed in the area. Consider a line divided into smaller partitions. The partitions tend to appear shorter than an isolated line of a length same as that of the partition. The reverse is also true. Consider two identical shapes. Imagine one to be kept among others of a comparatively larger size. Imagine the other to be kept in a group of relatively smaller sized ones. The former appears bigger. This is termed as the optical illusion of contrast. Thus optical illusions work on the principles of light and its interpretation by the eyes.