An optical illusion (also called a visual illusion) is characterized by visually perceived images that are deceptive or misleading. The information gathered by the eye is processed by the brain to give a percept that does not tally with a physical measurement of the stimulus source. There are two main types of illusion - physiological illusions that are the effects on the eyes and brain of excessive stimulation of a specific type - brightness, tilt, color, movement, and cognitive illusions where the eye and brain make unconscious inferences.
 

The Powers Hypnodisc. An effective yet inexpensive method of inducing hypnosis is with the aid of the hypnodisc spiral.

The spinning spiral will cause a series of optical illusions, causing immediate eye strain and fatigue. The subject feels that he is being drawn into a deep, dark revolving cone. By your suggestions of hypnotic sleep, you can place your subject in the somnambulistic state very easily. With some subjects, hypnosis will take place almost instantaneously. This technique is often employed in stage hypnotism.

The Fraser Spiral Illusion is an optical illusion that was first described by the British psychologist James Fraser in 1908.

The illusion is also known as the false spiral, or by its original name, the twisted cord illusion. The overlapping black arc segments appear to form a spiral; however, the arcs are a series of concentric circles.

The visual distortion is produced by combining a regular line pattern (the circles) with misaligned parts (the differently colored strands). Zöllner's illusion and the café wall illusion are based on a similar principle, like many other visual effects, in which a sequence of tilted elements causes the eye to perceive phantom twists and deviations.

The Reversed Spiral Illusion. It appears that concentric circles appear to be spirals, which rotate counterclockwise to approach the center. Here, the illusion is that there are not concentric circles but a spiral which rotate clockwise to approach the center. Thus, this illusion design shows the illusion of the spiral illusion.