What is an illusion?
The illusion is the misperception or misinterpretation of an individual that comes from a real object. E.g. the perception of a coil of a rope in darkness as a snake. It occurs because of confusion, eye movement, emotion, contrast perception, habits, defects of the sense organs, and a tendency towards the wholes.
Perception provides a clear and meaningful picture of the world around us and shapes our perceptual experience. However, sometimes the brain’s effort to organize sensations into coherent and accurate percepts fails. This is the cause of perceptual illusions in which normal perceptual processes produce perceptual misinterpretations. The perceptual illusion is a false perception. It is due to misinterpretations or misperceptions of stimuli that do not correspond to the sensations received by the eye or other senses.
For example, the grass looks greener on the other side of the fence. Because when we look directly at the grass of our own yard, we see both green grass and the dark brown soil in which it is growing. When we look at grass across the fence, however, we are not looking straight down, and therefore we do not see the brown soil. These colors blend. Hence, the grass on the other side of the fence is perceived as greener and more luscious than on our side.
Similarly, as we mentioned, the perception of a coil of a rope in darkness as a snake is another example of illusion. The stimulus is exactly like that coming from a snake and it is quite real and objective. Our past experiences, our fear, and the darkness help to perceive the rope as the snake because the rope and snake have similarities in structure except living and non-living stimulus.
Types of illusions
There are mainly two types of illusions: one is related to physical processes and another is related to cognitive processes.
Illusion Due To Physical Processes
These illusions are perceived without any existence of the stimulus. That is, things are not really there.
Mirages, for example, are an optical illusion in which the image of the distant object, as a ship, oasis, or water on the dry road ahead or is made to appear nearby. If it is caused by the refraction of light rays from the object through layers of air having different densities as a result of unequal temperature distributions. These illusions are something that falsely appears to be real.
Illusion Due To Cognitive Processes
Cognitive illusions occur in the presence of the stimulus but the individual simply misinterprets the situation or the stimulus. There are countless illusions related to cognitive processes, that can be broadly divided into two categories:
- Illusion of size
- The illusion of shape or area
Illusion of Size
Illusions of size occur because we perceptually distort the length of various lines. The theory of misapplied constancy suggests that we perceive some parts are farther away than others. There are several examples of size illusions. They are,
Muller Lyer illusion: Both arrowheads and the feather-headed lines are the same lengths. But the one with the feather looks longer or bigger. The symbols create the illusion.
The vertical line falsely appears taller than the horizontal line. But both lines are actually equal. The horizontal-vertical illusions stem from our tendency to perceive objects higher in our visual field as more distant.
The two-line segments are equal on the other figure, but the straight line outside of the circle appears bigger than the straight line within the circle.
Ebbinghaus illusion or illusion of contrast. The center circles are the same size, but one to the left looks larger because of its background.
Wundt illusion based on intersecting lines. The horizontal lines are parallel. We see the parallel lines as bent.
Hering illusion. The horizontal lines are parallel.
Eherensteing and Orbison illusion – The inscribed figures in g and h are perfectly symmetrical. The backgrounds in illusions can be viewed as three-dimensional (either concave or convex) thereby distorting the squire and circle superimposed on them.
Ponzo illusion – The two horizontal lines are of the same length, the upper one appears longer. It is an illusion based on linear perspective cues where parallel lines cover as they recede into the distance.
The Illusions of Shape or Area
The illusions of shape or area suggest that the geometrical stimulus is the other cause to mislead the perceptions. It includes,
Moon illusion – The moon looks bigger at the horizon (about 30 percent bigger) than at its highest point in the sky. This is because when the moon is near the horizon, we can see that it is farther away than trees, houses, and other objects. When it is overhead at its zenith, such cues are lacking.
Thus moon appears larger near the horizon because there are cues available that cause us to perceive that it is very far away. Another cause is that vertical distances are perceived as very long. The moon at the zenith is vertical, as a result, perceived smaller, because of distance cues.
Sander’s or Parallelogram illusion: The AC diagonal appears about the same length as BC, but is 16 percent shorter.
Poggendorff illusion: drawing in figure (see below) all three appear to be straight lines but it’s not so. Which line on the right is the conformation of the diagonal line on the left?
Ames Room: This distorted room designed by Adelbert Ames, appears to have a normal rectangular shape when viewed through a peephole with an eye. The girl in the near corner appears disproportionately large because we judge her size based on the tale’s assumption that Ame’s room is a regularly shaped room, with a normal ceiling height, and then the person in the right corner appears much longer than she is.