About Optical Illusions

EluZions.com Illusions : Ambiguous, Art, Chimera, Distortion, Dynamic, Impossible, Magic, Moire, Motion, Unstable • Books, Posters, UK Posters

Aside from magic tricks, the most spectacular visual illusions are those created in the brain itself. The most famous illusions in psychology are the Mueller-Lyer illusion (two lines with opposing arrowheads), the impossible triangle, and the Necker cube. Optical illusions were exploited in optical art, and by artists such as Escher and Dali. The variation in the apparent size of the Moon (larger when near the horizon) is an example of a natural illusion that is an optical phenomenon.

The brain takes cues from images received from the eyes to help it interpret what is being seen. Usually this is important for things like depth perception, but occasionally it leads us astray. The cues deceive us into thinking we see something that isn't true, or isn't even there. In the pages that follow, you will see that we can arrange marks on the paper or on the screen, that will fool your mind into seeing a false reality. Identical lines will appear to be different lengths, ghostly dark blobs appear where the screen or paper is white, black and white patterns appear to move when they can't, etc. They can be so convincing that you may have to check for your self that there's no trickery involved.

A popular dictum states that "seeing is believing". Most people understand that to be true enough, most of the time - but there are exceptions to this rule, as with most rules. Look at the picture here. You almost certainly can see some black dots appearing and disappearing randomly in the white discs. Are they really there, at all? You can check for yourself, in various ways, and determine that they are not actually present in the image itself. For example, cover up all but a small part of the picture. The black dots stop appearing. Or import the image into a paint program, and test the color values with the dropper tool - it will only register 'white'.

There are many more visual illusions. There is no trickery in the images themselves, the 'deception' occurs in the observer's visual system. Sometimes an explanation as to what is happening can be simple, sometimes it may be quite complex, and in some cases, no satisfactory explanation is known. However, the fact remains that the illusion may be extremely powerful, and some people may need convincing that what they're seeing isn't 'reality' as such, but an illusion.

Much of our normal visual perception is illusory in some sense. Further-away objects appear smaller than nearer objects. Mirrors show things where they can't be. Mirages decieve us about the distant landscape. Fish in ponds appear higher in the water than they are. The moon just above the horizon appears much larger than it should. The rising or setting sun looks redder than it really is. Parallel lines appear to converge to the horizon. Rainbows aren't tangible. Lenses alter the apparent size and distance of objects.

Most of these are physical illusions, caused by physical effects on the light as it travels from an object to our eyes, and we are not fooled by them. We know that distant objects are much larger than they appear to be. We know that objects seen in mirrors are actually in front of the glass, not behind it. For the most part, our brains and minds have adapted to these effects, and have learned to compensate for them in such a way as to correspond well to reality. We imagine the light bouncing off a mirror and can back-project it to its origin, where we find the source just as expected.

Perhaps the major exception to this ability to compensate for physical illusions, is magic. Magicians are sometimes referred to as illusionists, and we jokingly say that their art (or science!) is performed with 'smoke and mirrors'. Unless you want to believe that there are people who can suspend the laws of physics, then we are forced to conclude they they are in fact, clever opticians.

Aside from magic tricks, the most spectacular visual illusions are those created in the brain itself. In the pages that follow, you will see that we can arrange marks on the paper or on the screen, that will fool your mind into seeing a false reality. Identical lines will appear to be different lengths, ghostly dark blobs appear where the screen or paper is white, black and white patterns appear to move when they can't, etc. They can be so convincing that you may have to check for your self that there's no trickery involved.

So, what's going on here - why do these illusions occur, and why are they so convincing? Well, the specific explanation varies from case to case, and in many instances, either nobody is quite sure why, or the experts disagree. It's not as if you can open up the brain, like a computer case, and check the wiring! Psychologists can generally only perform controlled ethical experiments that isolate or eliminate the possible factors. But we can give an outline of the general case. The job of the eye-brain system is to create a model of reality for us, so that we can function in it. We need to find food and escape predators, for example.

However, reconstructing the real world from a pair of flat images is by no means trivial, and the brain does a substantial amount of processing and interpretation, in order to arrive at a most likely model of the viewed scene. This processing is based on experience and assumptions about the behaviour of reality, such as clues about how big or how far away things are. It's important to recognise the difference between a playful kitten and a hungry tiger! Some of the clues include: clarity vs. haziness; occlusion by intervening objects; convergence or divergence of bounding straight lines; etc.

These clues serve us well most of the time, but can be defeated by combining them in contradictory ways. The famous Muller-Lyer illusion (parallel lines with in- and out- ward facing arrowheads) is usually explained by interpreting the arrowheads as inside or outside views of rooms and buildings.

Another class of illusion arises from the physical structure of the eye itself, such as the plumbing of blood vessels into the eye, which creates a small area of the retina where there are no optical sensors - thus causing the well-known 'blind-spot' effect. We also have limitations on how quickly these sensors can adapt to changes in levels of illumination, giving rise to various 'after-image' and 'ghostly' effects.

Finally, we can also identify illusions that arise because there are almost no clues at all, or those that there are flatly contradict each other. In this genre we find ambiguous figures and impossible objects. The black or white wine glass is framed by almost nose-to-nose faces. The wire frame cube is seen from above or below without moving it or our viewpoint, and the solid-sided triangle has one side apparently approaching us, and one side apparently receding from us - yet they meet!

Curious-Minds.co.UK

IntelliQuest - Wild and Wacky Science £6.00	IntelliQuest - The Ultimate Interactive Space Quiz £6.00	IntelliQuest - Genius Lateral Thinking Puzzles £6.00
IntelliQuest - Horrid and Hideous History £6.00	IntelliQuest - Planet Earth Interactive Geography Quiz £6.00	IntelliQuest - Gross and Yucky Human Body £6.00