Reduction in the sharpness of the image or the distortion caused by the shape of the retina can both be contributing factors depending on the illusion. Other factors that can be responsible for optical illusions involve the ways in which the brain interprets visual images. These factors can include depth perception, relative size perception, or the misapplication of past experiences in visual interpretation. It has also been shown that the strength of the Mueller-Lyer illusion decreases with age, as a result of experience. In yet another theory to explain this illusion, scientists hypothesized that the length of the eye movement in order to scan each segment could result in the perception that one segment is longer than the other is.
Principles are yet to be found that are applicable to most optical illusions.
Optical illusions are usually caused by more than one physiological or cognitive factor and in most cases they are usually the result of an interrelationship of a number of them. The eye receives the images, but the brain interprets them. The result is sometimes that we see what we think we should see rather than the physical reality of the object. Although our overall experience teaches us to trust our senses, optical illusions remind us that what we see is also a product of our minds. The Psychology of Visual Illusions.