Illusions are wrong or incorrect perceptions. There is an external stimulation always present in such perceptions. These are not perceptions in the true sense, because in perception, the stimulation is correctly interpreted, whereas in illusions the stimulation is wrongly interpreted.

Therefore, illusions demonstrate that what we experience depends upon processes, which go far beyond the sensory input (Morgan et al, 1986). Illusions do not correspond to the objective situation as shown by physical measurement. There are illusions, which may concern most of the features of the physical world, such as distance, motion, shape, size, and direction, etc.

Illusions may be caused by a number of factors. There are stimulus factors, psychological factors, and cultural factors. The following figures demonstrate illusions, which are caused by stimulus factors.

Illusions may be caused due to psychological or personal factors. For example, if a person is having some problems in his eyes, or in his sense organs, he is likely to have illusions.

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Similarly, if a woman has lost her son in snakebite, she is likely to see a rope as a snake. In fact, in low light, all of us may perceive a rope as a snake because of our excessive fear of snakes. However, the role of personal factors in perception is marginal. Most illusions are universal phenomena.

Illusions are also caused by cultural factors. The Zulu people of South Africa do not suffer from Muller-Lyer illusions
or the Sander Parallelogram illusions as much as the people of the carpentered-world. The Zulu people spend most of their time outdoors and are less familiar with carpentered objects having rectangular patterns. Their huts, their cattle, pen, etc. are circular in nature. The circle also is an important aspect of their religious activities. Therefore, they are less likely to perceive Muller-Lyer and Sander illusions because of their lack of experience with rectangular objects.

A number of explanations have been used to account for different types of illusions. Important among them are the eye-movement theory, the perspective theory, and the inappropriate constancy scaling theory. The eye-movement theory suggests that the feather-headed line in the Muller-Lyer illusion looks longer because our eyes move a longer distance to see the line than the arrow-headed line.

Therefore, the feather-headed line occupies wider territory in the retina for which it is perceived longer than the arrow-headed line. But according to the perspective theory, the pictures represent some depth cues for which they are interpreted differently.

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For example, the feather-headed line represents the inside corner of a house which moves away from us and the arrow-headed line represents the outside corner of a house which is extending towards us. We know that an object, which is at a distant place, should look smaller. The two lines are actually of equal length, but one is perceived as moving away from us i.e., it is at a distance but having same retinal image like the other line, which is moving towards us. Therefore, the feather-headed line is perceived as longer.

The moon illusion has been described in terms of the perspective theory. Near the horizon, the moon looks bigger than the moon in the zenith i.e., when it is over our head in the open sky. The horizontal moon is seen with other objects such as trees, mountains, etc. The zenith moon, on the other hand, is seen in the empty sky. That it is just over our head means we perceive it nearer in space. The horizontal moon is perceived to be at a far away place. But the retinal images of the moon remain the same. We, therefore, interpret the retinal image of the horizontal moon to be larger in size because it is farther away.

The most appropriate theory to explain illusions is the inappropriate constancy scaling theory developed by Gregory (1963). In simple language, it could be stated that when perceptual constancy fails, illusions take place. To maintain perceptual constancy, the brain analyses several information just like a computer. These are: the distance of the object, retinal size, and the nature and characteristics of the object, several other depth cues, etc. Taking all these into consideration, the brain takes a decision with regard to the probable size and distance of the object. But at times, the brain does not get sufficient information to arrive at a definite conclusion. In such cases, constancy is not maintained and illusions take place. However, there is not a single explanation for all types of illusions.