How do our eyes and brains create illusions? Here and in the next seven pages John Stringer shows your pupils four different ways.
Ask your pupils to look at this famous sporting icon. Who is it? It's footballer Michael Owen. Immediately recognisable. But something is not quite right. Now turn him over and show him to your pupils.
1. THE ART OF TRICKS
The picture of the footballer, Michael Owen, on the page 31 looks all wrong upside down. Why?
The reason is that the picture has been tampered with. But that tampering would not work if it weren't for the extraordinary interpretational powers of our brains.
Humans are used to seeing faces. In fact we're so good at it that research findings suggest that a series of human faces might make a more memorable personal identification number for our bank card than numerals.
So when we look at a face, we see what we expect to see.
We don't see many upside-down faces. So when we see one, we don't notice that the eyes and mouth are the right way up at first. But turn the face over, and it looks all wrong. You know what faces should look like, and upside-down eyes and mouth scream at you.
This project demonstrates how artists and photographers use tricks like this to deceive us, including how they can also take a flat surface and give it depth.
THINGS TO DO
What's wrong with Marilyn?
She has green lips. But look at her picture for half a minute. Relax your eyes as if you were soaking up the picture - which in a way, you are. Then look quickly at the white paper. What colour are her lips now?
What you are seeing is an effect created by your eyes.
Your colour receptors can "tire" of seeing green. Then when you look at the white paper, they give you the "opposite" of green - red (or at least pink).
Some people have difficulty in seeing this effect.
A few may have colour vision deficiency - but this is not a test for colour blindness.
If you get red with green, what will you get with red?
Cut out a square of red paper and put it on a sheet of white paper. Look at this - and then away. What do you think you will get with blue - or yellow? Try it.
What will you get with a blue square on a yellow background? Experiment!
Pictures painted in "opposite" colours like these dazzle - and seem to move.
Try a painting using only bright red and green paints.
2. THE ART OF PERSPECTIVE
On pages 34 and 35 you will find a painting of St Peter's in Rome, painted by Giovanni Paolo Panini between the years 1746 and 1754 (shown right).
Your eye is drawn down the long hall. You are looking into the distance. Panini was a master of using perspective to give the illusion of depth and distance. You almost feel you could walk into the painting.
Perspective drawing was discovered in Europe in the early 1400s. Before then artists painted all figures - foreground and background - the same size. Their pictures were flat and unreal. Panini's picture also uses shading and shadow cleverly, to give three dimensions to his figures.
THINGS TO DO
* Try shading pictures. Can you shade a picture of a box, or a cylinder, or even a ball, so that it appears to be three dimensional.
* You can put perspective in your drawings. Try this: a) Take a piece of paper and draw a horizontal line across the middle of it. This is the imaginary skyline - the horizon.
b) Put a dot in the middle of the line. This is the vanishing point. Now use your ruler to draw two lines on the bottom of the picture. This gives you a road.
c) Draw another line at the top of the picture. Between that line and the road, draw in some lampposts or some telegraph poles. Remember that they are upright, and must stay upright as they vanish into the distance.
d)Now use the same technique to put houses on the other side of the road; people on the road; trees in the background.
See how you are adding the illusion of depth? It helps that you have two eyes!
* At key stage 1, pupils should be taught in science "about the senses that enable humans and other animals to be aware of the world around them; in art and design, they should be taught about colour, pattern and texture, line and tone, shape, form and space; differences and similarities in the work of artists... in different times"; in information and communication technology they should use images to develop their ideas.
* At key stage 2, pupils should be taught in science about the role the subject has played in the development of many useful things, and they should carry out a range of investigations in art and design, they should use a variety of methods and approaches to design and make images; in ICT they should bring together, organise and reorganise images.
3. THE ART OF PATTERNS
This picture on the left is flat-printed on paper. But your brain cannot accept this. Because of the shading in it and contrasting colours your brain tries to see it in three dimensions. Look longer, and the "roof" surfaces become "ceilings", turning the picture inside-out.
We know that our brains work very quickly. They give us an instant picture rather than an accurate one. This is why we can "see" things that are not there. A plastic bag blown across the road becomes - for a moment - a white cat. We can expect to see a cat dash across a road (they often do) and so our brain tells us - instantly - that this is the most likely explanation. A moment later we see a shopping bag.
When you look at this picture, your brain starts searching for a pattern - or a picture - that is not there. But it is able to fill the space.
The optic nerve emerges from the back of your eyeball and where nerve and eyeball meet, there are no light receptors. You have a blind spot. When you look around you, you are not aware of it. Like a computer photoshop programme, your brain takes a piece of neighbouring picture and fills the gap.
* Magic Eye
The picture on the right is a stereogram - otherwise known as a "magic eye" picture. Look at it, stare blankly, and move the book backwards and forwards. An image will come into view.
Stereograms take advantage of the fact that we have two eyes. Our brains have learned to pay close attention to things that are identical in both left and right visual fields. The brain can transform a flat page into a three-dimensional pattern. It can turn random dots into vivid pictures, without the use of special glasses or computers.
THINGS TO DO
* Draw a black dot and cross on paper, roughly 10 cm apart and on the same level. Look at them. Close your left eye, look at the dot with your right, and bring them towards you. The cross will vanish. It is falling on the eye's blind spot. Now close your right eye and look at the cross with your left. The dot will vanish.
* With white paper and black paint, colour in alternate squares. Rule the squares into triangles and colour in alternate triangles. Look at your patterns. Your brain will start searching for a pattern - or a picture - that is not there.
THINGS TO DO
You can't make stereograms like the picture on the right. But you can find out more about your three-dimensional vision. Try this:
* Almost everybody has one eye that is stronger than the other. We use this dominant eye unconsciously for judging distances. To find out which is your dominant eye, hold up a finger. Using both eyes, line the finger up with a corner of the room. Keep the finger still, but close each eye in turn. The finger will appear to "jump". It jumps furthest towards the most dominant eye, which you unconsciously used for the lining-up.
* Your brain tries to make sense of the most confusing images. Look into the distance; then put your hands near your face and bring your index fingertips together. As they touch, you will see the fingertips more than once. In an effort to understand what is going on, your brain has linked the left and right images to make a floating finger. Your brain is always looking for patterns.
4. THE ART OF TROMPE-L'OEIL
Trompe-l'oeil is the art of painting so that the objects appear to be real. A window painted on a wall, for example, can be so real that you feel you could open it and lean out. A master of the art was Cornelius Gijsbrechts (pronounced Guys-breckt). His work, seen right, is showing at the National Gallery in London until May 1. (020 7747 2885). Admission is free.
The National Gallery is holding workshops for visiting families. You can become part of a living trompe-l'oeil on one of their Science of Art and Illusion days. They have a new education centre too, that welcomes school parties - though they expect most school visitors will be there to see the paintings.
* TEACHER NOTES
The images in this project encourage children to think about what they see and the way the brain interprets images to make sense of the world.
Pupils enjoy optical illusions; and developments in information and communication technology extend the opportunities for developing a range of images and illusions. Many schools have cameras so children can store images on computer. Programmes like Adobe Photoshop and Kai's Power GOO allow them to enhance, to develop and even distort images. Restricting the range of colours can create striking effects. Using only black-and-white, or two contrasting colours, can produce illusions of depth and movement.