Illusion, an impression of a real stimulus, received through one of the senses, which does not agree with other sense impressions or with objective tests. For example, a straight stick half submerged in a glass of water appears to be bent at the water line. However, if the observer runs a finger along it, the stick will feel straight. The bent appearance is called an optical illusion. Illusions may be perceived by the other senses, also, but optical illusions are perhaps the most common. Experiments show that many animals, including birds, perceive illusions.

Illusions are normal errors in perception (normal in the sense that they occur to all observers in the same way under the same circumstances). They are to be distinguished from hallucinations and delusions, which are abnormal experiences unique to the individual.

Some illusions are caused by physical factors. The apparent bending of the half-submerged stick is an example of this kind of illusion. The eye actually sees a bend in the stick because of the refraction (bending) of light rays as they pass from the water into the air. Other illusions may depend wholly or in part upon psychological factors. For example, if a person simultaneously picks up a large box and a much smaller one of the same weight, the larger box will seem the lighter. This effect is due to the expectation that the larger box will be heavier and, when it is not, an exaggeration of its lightness.

Stage magicians often employ psychological illusions in performing their tricks. The ventriloquist's ability to throw his voice is in part due to psychological effect, because he makes no observable lip movements.

Examples of Illusions
Apparent Movement

This illusion is produced by presenting images in succession at regular intervals. It may be observed in motion pictures and in electric signs that seem to show a moving object. Motion pictures are made up of a number of still pictures projected on a screen at a fast rate in proper sequence. Apparent movement in electric signs is accomplished by successively turning a number of lights on and off. The illusion depends for its effect on the fact that the eye continues to see images for a fraction of time after they have disappeared and blends successive images together.

Moon Illusion

The moon appears larger on the horizon than when overhead. Various theories have been advanced to explain this illusion. One theory relates the effect to the body posture and eye movements of the observer. Another explanation, known since the days of the ancient Greeks and given support in modern times, is that the moon appears larger on the horizon in comparison with objects of the landscape lying between the moon and the observer. When the moon is higher in the sky, there is nothing to give it relative size in the eye of the observer.

Doppler Effect

As a locomotive approaches an observer, the tone of its horn is heard as increasingly higher in pitch. As the locomotive moves away, the horn is heard as increasingly lower in pitch. A comparable effect may be observed optically. As a star approaches the earth, it appears violet; as it moves away, it appears red.

This illusion was named after the 19th-century Austrian physicist, Christian J. Doppler, who discovered its cause. The number of sound or light waves reaching the ear or eye in a given interval of time determines the tone or color perceived. As the object giving off the waves approaches the observer, the waves reach him with increasing frequency. As the object moves away, the waves reach him with decreasing frequency.

Aristotle Illusion

An easily demonstrated illusion of touch is believed to have been discovered by the ancient Greek philosopher Aristotle. It may be produced by crossing the middle finger over the index finger and placing the tips of both fingers on a small, round object, such as a marble. The marble will be felt as two marbles.

Other Illusions

There are a great number of geometric and color illusions, many of them essential to the work of artists, architects, landscape architects, and interior decorators.

Vertical lines appear longer than horizontal ones, and unbroken lines appear longer than broken ones. In the Müller-Lyer illusion, a straight line enclosed by two small inward-turned lines appears shorter than a line of equal length enclosed by outwardturned lines. Perspective, the representation of depth and distance on a flat surface, depends upon a combination of geometric illusions.

Most color illusions are produced by the effect of one color upon another, or upon another value of the same color. For example, a small area of blue on a purple background appears greenish. The same blue on a green background appears slightly purple. In each case, the blue seems to be tinged with the primary color that is not present.