Our perception of the stimulus world is not always accurate. In certain occasions, the sensory information may be incorrectly interpreted. Such cases are known as illusions. A visual illusion occurs when two objects produce exactly the same retinal image, but are perceived as different images.
Illusions are errors in perception, but they are not abnormal. They are experienced by normal human beings as well as by birds and animals. They can also be experienced by the stimulation of any of our senses. More than 200 different types of illusions have been discovered.
All of us experience moon illusion. In the evening, the moon near the horizon looks bigger in size than in the midnight, when it is over our head. Similarly, the rising sun looks bigger than the afternoon sun. Sun or moon does not change the size as it moves from the horizon.
In other words, in such situations, our sense organs deceive us. The stimulus is wrongly interpreted. This wrong interpretation of sensory information is called illusion. We have heard people bet that they have seen ghosts. This is a false interpretation of sensory information often called as hallucination. Illusions and hallucinations are two kinds of errors in perception. Both are distortions.