Anais Nin says, "We don't see things as they are, we see them as we are!" This is a fact, which makes itself evident in everyday life. We see different people having different interpretations of the same subject; we see different people think differently and more importantly, we see different people see differently! Does this puzzle you? Imagine the sun, rises in the East and sets in the West. Doesn't it appear to have traversed across the sky, when it has actually not? Mirrors in a room make the room appear larger than it is. This is due to reflection of light as perceived by our eyes. Human eye is actually a wonderful optical mechanism. Light falling on an object gets reflected to form an inverted image on the retina of the human eye. The brain interprets the image and we perceive the object. Actually illusion is the only reality, as whatever we see, we think real and not everything that we see is a reality! Illusions are a result of mistaken judgment or erroneous vision. The error in the way of seeing things results in optical illusions. It is the way we perceive things that matters. It is the way our eyes perceive and brain interprets, that affects our understanding of things around us. The intricate yet interesting design of the optical mechanism in our eyes is one of the basic causes of optical illusions. Human eye is so constructed that the two types of nerve cells, namely, the cones and the rods, are distinctly placed on and around our retina. The cone cells detect color and the rod cells detect the low-light contrasts and the two types of nerve cells together convert light into neural signals. The optic nerve carries the signals to the brain. On the edges of the retina, there are fewer cones and more rods while the center of the retina has only cones. Thus, seeing things from the corner of the eyes can prove deceptive. For a clear picture, we need to turn the eye as the sharpest images forms in the center of the retina. An excessive exposure to alternating patterns, brightness, tilt or a particular color may affect the perceptions by our eyes. Continuous stimulation of specific regions of the brain can result in a psychological imbalance, thus affecting subsequent visual perceptions. Repeated exposure of a certain color of a certain amount of brightness tires our eyes, due to which we may perceive things differently. Thus tiring of eyes following an excessive exposure to a certain kind of visual, can cause optical illusions. Some artists incorporate an illusive effect in their paintings. Their works carry an intentional ambiguity about what is depicted in the picture. Distortions in size, shape and curvature can affect visual perception. Sometimes, objects, which cannot possibly exist, are made to appear in certain pictures. Here, the artist's knack of bringing about an illusive appearance in his/her paintings influences perceptions by the human eye. Illusions can be caused by certain medical syndromes like schizophrenia. Those affected by certain neuropsychological diseases may experience illusions in most of the things they see around them. It is due to their brain disorder, that they mistake objects; they perceive objects differently than what they actually are. Similarly defects in the lenses lead to optical illusions. Spherical or chromatic aberrations or abnormalities like astigmatism and eccentricity can result in optical illusions. Optical illusions such as 'advancing' and 'retiring' colors are a result of chromatic aberrations arising out of the differences in the angles of refraction and reflection of different colors. Differences in shades of a color can give depths to flat surfaces. The effort involved in raising the eyes is greater than that in turning them in a horizontal plane. Owing to this difference in the effort, our eyes perceive vertical distances as being greater than horizontal ones of equal magnitude. When our eyelids are almost closed, their movement makes objects appear to be moving. The concentration of objects placed in a particular area in space, influences the perceptions of distance and the sizes of objects. Imagine a cube placed among other larger sized cubes and an identical cube placed among relatively smaller sized cubes. The former will appear smaller than the latter, though both the cubes are identical in size. After all, we don't always know what we see, but we tend to see what we know. Briefly, the behavior of light, the perception of light by the eyes and its interpretation by the brain are the main causes of optical illusions.