An optical illusion is an error of visual perception caused by one of two things - either a defect in the visual mechanism, or an error of judgement. Intellect can play a part, enabling some viewers to spot an illusion.
Fly Spy Amazing Optical Sheet (far left)- This is a viewing sheet for reproducing the effect of an insect's compound eye. It was made in Taiwan in 1994. Insects can see sharply for only 2-3 feet and cannot focus. This is a child's educational toy but more sophisticated replicators are used in optometric teaching so that future optometrists can understand what living with various eye conditions is like.
"Magic Winky-Blinky Eyes" (1998, near left). Moving the sheet causes the eyelids to close. According to the packaging 'Winky Technology' was introduced in the 1940s/50s - a picture composed of two or more images is placed beneath a plastic ribbing in such a way that only one image can be clearly seen from any given angle. The eyes were in the form of stickers meant to be peeled off and worn to amuse your friends.
The illusion makes use of two devices to suggest distance - the perspective lines receding to a vanishing point and the use of shading. Because these features cause the eye to think that the darkest elephant is nearer, we assume it will appear largest. The fact that the lightest shade of elephant appears larger fools us into thinking it is actually a bigger elephant.
An optometrist would explain the illusion this way: The Necker cube is a reversible figure in which the two alternative depth interpretations do not involve a change in the apparent sizes of the 'near 'and 'far' faces.
The rest of the illusions on this page are reproduced from the Norville Optical Company's corporate calendar for the year 1980. These were not just a bit of light relief for the optician's wall but a serious educational tool.
There are two types of distortion illusion: Distortion due to background and self-distortion.
The diagrams to the left are examples of distortion illusions produced by the background.
The red square in Figure 1 and the red circle in Figure 2 are striking examples of this type of visual distortion. They are geometrically perfect, but the concentric rings or the radiating lines conspire to make us think they are not.
Angles play an important part in the production of optical illusions. The diagram on the right demonstrates the effect on parallel lines by different types of radiating background.
In Figure 3 the red parallel lines are distorted by the converging black lines of radiation behind them. Because the centre of radiation (i.e. the source from which the lines stem...in this case a twin source) lies outside the parallel lines, a 'bow-in' effect is produced.
In Figure 4 the centre of radiation lies between the parallel lines and so a 'bow-out' effect is caused.
Finally Figures 5 and 6 are two classic illustrations of background-producing distortion. Although the long lines are, in fact, parallel the small cross-hatchings produce the illusion of the lines appearing to converge or diverge alternately. Even if you try to let your eyes lose focus the red lines still won't appear parallel.
Go on, give it a go!
Can you think of any instances where this illusion occurs in everyday life?
Our next three illusions are all examples of self-distortion illusions and may be more familiar to you. They often feature in children's books though they are rarely afforded their proper names.
Figure 7 is known as the Muller-Lyer arrow illusion and was first devised in 1889. The vertical lines (shafts of the arrows) are exactly the same length but the one with the outward pointing arrow heads appears longer to most people.
Figure 8 is known as the Ponzo Figure, named after the Italian artist and psychologist Mario Ponzo (1882-1960). It consists of two converging lines of equal length between which are two horizontal and parallel lines, also of equal length. Because the upper of these lines occupies a narrower space it appears longer to most people.
Figure 9 features two geometric shapes. Would you believe that the top hoizontal line of each shape is the same length? The contrast in the angles of the two shapes help to cause the illusion.
The elephants cartoon above was an example of a prespective illusion. Here are some more.
Figures 12 and 13 are known as Reversible Cubes. The circles comprise a pattern of multiple cubes. In each cube the white surface can be viewed as either the upper surface or the lower surface. Once you've seen it as one or the other it can be difficult to switch your brain into viewing it the other way. Occasionally the effect of perspective disappears and the figure seems to consist of a series of parallel lines of white diamond shapes.Notice how this phenomenon is more marked on the figure reproduced in black only. The addition of a second colour can help us make greater 'sense' of an image.
Perspective is very influential in helping is to judge the relative height of objects. The converging lines of Figure 14 suggest to us a scene with depth. From experience our brains know to make an allowance for a decrease in size with an increase in distance. In fact the three girls are all drawn the same size.
Our next illusion presents a seemingly impossible situation. Naturally we assume that the room is rectangular, but this is not the case. the diagram to the side of Figure 15 shows the true shape of the floor plan. This illusion was devised in 1946 by an American psychologist and vision scientist, Adelbert Ames jr (1880-1955). The far wall is in fact receding from the viewer. By drawing the walls, door and windows so as to give the same retinal image as a normal room, the viewer is fooled into thinking the two boys are disproportionate sizes whereas the boy holding the ball is in fact standing further away from us than the boy holding the toy yacht.
Ames Rooms have featured in various films and television shows, especially when the desire has been to create a fantasy world. The British soft rock band Status Quo featured a photograph of themselves in such a room on the cover of their LP 'On the Level' (1975).
The darker the colour the more pronounced the effect. So in Figure 17 the length of each lozenge equals the distance between its outer edge and the tip of the central star but this is very hard to believe unless you measure it.
In Figure 18 the grey ring is of even tone throughout but to most people it looks lighter on the left because of the dark contrasting background.
Figure 19 is perhaps harder to make out and easier to detect from a printed page than from a computer screen. When correctly reproduced the square appears to have dark areas at the points where the white bars cross it. You see dotted after-images. The illusion is sometimes more noticeable if viewed with averted gaze. By focusing on just one crossing point the illusion appears much weaker and may not be detected at all by some viewers. The illusion is called Hermann's Grid and was first presented in 1870.
By skilful drawing these pictures appear to represent objects. The eye starts trying to make sense of them and realises that they cannot be interpreted properly in three dimensions.Nevertheless, if you look away for a few moments and then return to the figures the illusion starts all over again.
Figure 22 is called the Penrose Impossible Triangle, devised by the mathematician Roger Penrose in the 1950s. It features prominently in the works of the Dutch graphic artist M. C. Escher (1898-1972) whose earlier works may have inspired Penrose.
Impossible figures can be a source of great amusement. Our visual perception system will continue trying to build a three-dimensional form, but as the two-dimensional information is not compatible we will never reach a solution.