Check out the image above. What do you think? Are the two horizontal lines parallel to each other or are will they eventually come to a point? Your visual system might not be telling you the truth!
Cut about 10 identical strips of one colour card around 80 cm long and about 5 cm wide. Cut about 80 squares of the other colour card, exactly the same width as the strips. Glue the squares onto the strips as far apart as their own size.
Glue the matchsticks end to end onto the board to make 11 rows a little more than 5 cm apart. Slide the checkered strips in between the matchsticks and tape pieces of paper so they hang over the ends. Invite students to pull or push the strips and watch what happens to the overall effect. When the squares are lined up vertically everything appears normal but changes to this alignment have a dramatic impact on our perception of the strips themselves. Suddenly they seem to be anything but parallel. In fact, they can start to look decidedly wonky!
This intriguing phenomenon is called the Café Wall Illusion and has a tendency to keep people entertained for ages.
Which of the centre circles seems larger? Try drawing your own versions, experimenting with other shapes and combinations to test whether the shape or size comparisons contribute more to this illusion.
Try this one at home. Take a paper plate and cut off the rim. Then, cut a segment which makes up less than a quarter of the rim and use this as a template to cut another, identical segment. Arrange the two arcs next to each other. Even though you've cut them both the same size, one arc will seem much bigger than the other.
You can also try an interactive online version of this – Questacon Cards.
These illusions all incorporate similar features of our visual systems. Our brains are programmed to judge by comparison — we use ‘relativity’ a great deal in our visual processing. In these illusions, our visual systems jump to inaccurate conclusions on the basis of nearby clues.
These size, shape and direction distortions are very common illusions and occur as easily with any shape or combination. Can you develop a misleading design of your own?
Which of the two squares looks bigger? Measure both and see if you're right! Most people say the white square is larger than the black but they are actually the same size. If you've got some red, white and blue paper, try to cut them into equal strips to create a copy of the French flag, but don't use a ruler. Once you're finished, measure each band — most people find that they have made the white band narrower than the other two, even though all three bands seem the same width. The real French flag has the white section made slighter narrower so that it will appear to be the same!
This illusion is an example of the irradiation effect in which light colours appear larger than dark objects of the same size. Dark objects reflect less light energy (which is why dark cars feel hotter in the sun than light coloured ones) but this is not the illusion. Our brains notice the difference in light energy and make the assumption that it is due to a difference in size. This principle is used a great deal in interior decoration and fashion design.
Try sharpening a lead pencil a few times, and keep the ‘lead’ part of the shavings. Then, rub these shavings on paper to create two ball-shaped smudges which become lighter towards the edge. Use a black felt pen to draw a black dot in the middle of one of the smudges. Stare at the dot-less smudge for one minute and see if anything happens. Likely, nothing will. Now, stare at the black dot for one minute without moving your eyes. At the end of the minute, move your eyes slightly but keep looking at the black dot. At first the smudge will start to disappear altogether, making it seem as if the black dot was surrounded by white paper. Once you move you eyes, however, the smudge will miraculously appear again.
This illusion demonstrates a feature of human vision which scientists call a ‘stabilised image system’. The brain likes to concentrate on changes. It compensates for tiny unconscious eye movements so that an unchanging stimulus will keep being detected by exactly the same part of the retina. The smudge is not a strong stimulation for human vision because it does not have a clear boundary. In comparison to the black dot, our brain ‘decides’ the smudge is irrelevant. A large enough eye movement means that light from the smudge is now falling on a different part of the retina so we can see it all over again.
Freehand draw a horizontal line on a piece of paper, then draw a vertical line the same length so that it meets the middle of the first line. Now, measure the lines and see how accurate you were. Ask some friends to try it too. Did everyone draw their horizontal lines too short?
Most people overestimate the length of vertical lines. This is thought to be because side to side eye movements are easier than up and down ones and it is the extra effort required to judge a vertical distance which tricks our brain into perceiving the line as longer than it really is.
Hold a print out of the image at arms length and cover one eye. Now, focus your open eye on the spider and gradually bring the picture closer towards your face, staying focused on the spider. At some point the blank spot disappears and it seems as if the web is complete.
Every eye has a blind spot — the part of the retina where the nerve fibres are gathered together to form the optic nerve for communication with the brain. When the image of the blank ‘hole’ in the centre coincides with the eye’s blind spot, our brains ‘fill in’ the image with what it expects to be there. Our eyes do this all the time without us noticing — there is always a blind spot yet we never see any blank areas in our vision - the brain just makes one of its educated guesses again.