One logical reason for optical illusions is mistaken judgment. An error in the estimation of distance or depth can cause optical illusions. If objects of contrasting colors are placed beside each other, our eyes mistake one color for another. Different colors absorb different amounts of light. This affects the perception, leading to optical illusions.
According to the Lipps' principle of mechanical aesthetic unity, our eyes give every space the form of a living being and unconsciously consider some mechanical forces acting. Wundt suggests that the principles of retinal image formation are a major reason behind the occurrence of optical illusions.
When an object reflects the light falling on it, the eye forms an inverted image on the retina. The brain interprets this image and derives meaning out of it. The structure and functioning of the eyes are one of the important causes of optical illusions.
One's eyes may possess abnormalities like astigmatism and eccentricity. Lenses possess defects such as chromatic and spherical aberrations. These phenomena are responsible for optical illusions. Certain neuropsychological diseases can cause optical illusions.
Judgments about the direction and distance are not accurate, when our eyes are closed. It is normal for a human eye to perceive vertical distances as greater than horizontal ones of equal magnitudes. Perhaps, optical illusions occur because we don't always know what we see, but we tend to see what we know.