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There are many types of optical illusion but, in every case, the brain is being fooled by what the eyes see. In a short blog such as this I can only show a small set of examples from a limited number of categories but, if you are as fascinated by these as I am, a short web search will reveal countless more. It’s also the case that some optical illusions can only be created using paper, and won’t work on-screen.

Illusory figures

An illusory figure is created when background figures, often with carefully placed gaps, create the strong illusion of a figure in the foreground – but the figure isn’t actually there! – your brain is “filling in” the gaps. In Illusion 1, the circle sectors and the arrowheads are real, but the white triangle isn’t – it even appears to be whiter than the background.

Illusion 1

Shape distortion

There are many ways in which shapes as simple as straight lines can be made to appear what they are not. One of the most common is to make parallel lines appear non-parallel or even, as in the next example, curved. This one is called the cafe wall figure where a mosaic of black and white square tiles, vertically offset, make the lines between the tiles look distinctly wavy. They are, in fact, straight and parallel.

Illusion 2

If you don’t believe the lines are parallel, try measuring the distance between them at different points.

Length distortion

Similar to shape distortion, our perception of lengths can be very easily fooled. In the first example, the vertical and horizontal lines are the same length, but because our horizontally aligned eyes scan more easily side-to-side, the vertical line is interpreted by the brain to be longer.

Illusion 3

The next example is a version of the Muller-Lyon Illusion. The two lines are the same length, but the one where the arrowheads point outwards is generally judged to be about 25% longer – and there’s nothing you can do to alter the perception. Only the use of a ruler will prove they are the same length.

Illusion 4

Impossible Figures

Most impossible figures derive from work in the 1950s by two British psychologists, L. S. Penrose and R. Penrose, who also drew on work by the Dutch artist M. C. Escher. Illusion 5 illustrates an impossible cube: at first sight the framework appears to be a simple cube but a quick inspection shows that the back left vertical is simultaneously at the front and at the back of the structure. Clearly, such a framework would be impossible to construct.

Illusion 5

And in illusion 6 we have an example of the “three pegs becomes two” figure, in this case known as the “devil’s tuning fork!” The right-hand end appears to have two arms, but this becomes three at the left-hand end. It’s a really frustrating figure to try and make sense of!

Illusion 6

Ambiguous Figures

Ambiguous figures consist of images which present at least two different figures, although you may not see both initially. It’s a fascinating aspect that, once you have seen both figures, it’s impossible to focus on one without the other popping into your vision.

My first example is the famous “two faces, or a vase?” first published in about 1915. Concentrate on the black, and you’ll see two faces; then look at the white, and you’ll see a vase.

Illusion 7

Illusion 8 is called: “Old woman or young girl?” You may find it harder to see both images at first, but try covering the right half, and you should see the young girl in profile, looking away over her right shoulder; now cover the left third or so, and you’ll see the old woman, with her large nose and jutting chin.

Illusion 8

Figure-ground illusion

Here we see what appears to be a jumble of shapes or patterns, but you need to look at the image in a different way to make sense of it. Can you work out what illusion 9 illustrates before I explain it to you?

Illusion 9

Your eyes tend to concentrate on the closed shapes – which are indeed meaningless. Now concentrate on the spaces in between, and you should see the word WEST appear. If you still find it hard, try putting a ruler along the bottom or top edges (or both) to enclose the spaces, and it should become clear.

Picture credits: Several pictures taken from the website of the University of Glasgow Centre for Perceptual Experience.