PERCEPTION AND ILLUSIONS - Science World Resources

Illusions 

PERCEPTION AND ILLUSIONS 

Is seeing believing 

OBJECTIVES: 

Students will be able to: 

• Understand that visual 

information is processed by 

the brain 

• Explain that the brain bases 

its interpretations on past 

experiences 

• Describe how optical 

illusions occur 

CURRICULUM 

CONNECTIONS 

BY GRADE: 

K. Physical Science 

2. Physical Science 

4. Physical Science 

5. Life Science 

LIST OF ACTIVITIES: 

Optical Illusions 

• Image Decoder 

• Do As I Say, Not As I do! 

• Seeing Spots 

• Random Dot Stereograms 

• Necker Cube 

• Size Perception 

• What Do You See 

• Stroop Effect 

• Canadian Flag After-Image 

• Benham’s Disk (make & take) 

• Thaumatrope (make & take) 

• Stroboscope (make & take) 

Auditory Illusions 

• P-O-T-S 

• Speech-to-Song Illusion 

Tactile Illusions 

• Numb Fingers 

• Finger-to-Elbow 

• Fake Hand 

• Comb Dot 

Introduction 

It is often said that seeing is believing, but can we really trust what our eyes are telling 

us In this resource, students explore optical illusions and how our brain strives to make 

sense of the information it receives. 

Background 

An optical illusion occurs when the brain perceives something that does not reflect 

reality. In fact, the word illusion comes from the Latin verb illudere or "to mock." 

Optical illusions can result in three ways: 

1. Your brain makes faulty assumptions based on patterns you have experienced in the 

past (cognitive illusion), 

2. Your eyes have structural quirks that misinterpret visual cues (physical illusion), or 

3. Your eyes or brain react to something they’ve been observing for a long time 

(physiological illusion). 

Your eyes are optical instruments, like a camera, microscope or telescope, but they 

adjust automatically. Your eyes can tell different colours apart, they can adapt very 

quickly to variations in the amount of light they're receiving, and they can focus 

themselves automatically. 

Iris 

Pupil 

Lens 

Retina 

Optic Nerve 

to Brain 

Cones on retina 

The iris controls the quantity of light entering the eye. The lens focuses the light to form 

an upside-down image on the retina. Right at the beginning of your development as a 

baby, your brain learned to turn the reversed image right-side up. 

The back wall of the human eye (retina) is lined with two kinds of receptors for 

distinguishing light: there are 12 million rod cells and 7 million cone cells. 

Rods respond to all wavelengths of visible light in low light conditions. They allow us 

to see objects when the lights are dim. Cones, on the other hand, respond to certain 

wavelengths, sending colour information to the brain. 

RESOURCES.SCIENCEWORLD.CA

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There are 3 different types of cone receptors for coloured light: one is most sensitive 

to red light, one to green light, and one to blue light. With these three colour receptors 

we are able to perceive more than a million different shades of colour. Cones are less 

sensitive than rods, requiring higher levels of light. This is why it is difficult to make out 

the colour of objects in low light conditions. 

Ghostly coloured or squirmy after-images can appear when rod or cone cells get tired. An 

image that your eye collects is just the first step in the creation of meaning. Where the 

information from your senses is unclear, incomplete or seemingly backwards, your brain 

works to understand the world by filling in gaps, ignoring data or making decisions, 

usually based on your previous experience. This interpretation may not reflect reality: it 

can result in an illusion. 

Although this resource emphasizes optical illusions, both tactile and auditory illusions also 

result from the brain’s faulty interpretation of data received from these senses. To reinforce 

this concept, tactile and auditory illusions have been included as activities and extensions. 

Teacher’s Tip: 

We suggest that you rotate between the different types of illusions, ideally 

setting up stations around the classroom for students to visit in turn. 


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Vocabulary 

Auditory illusion: an illusion involving the brain’s misinterpretation of sound signals. 

Lateral inhibition: when one cell reduces the activity of a neighbouring cell. 

Mirage: an optical illusion where light rays are refracted, or ‘bent’, to produce a displaced image of 

faraway objects. 

Optical illusion: an illusion involving the brain’s misinterpretation of visual signals. 

Proprioception: our sense of the relative positioning of the different parts of our body (i.e. our 

internal “map”). 

Sensory Receptor: a sensory nerve ending that responds to a stimulus. 

Tactile illusion: an illusion involving the brain’s misinterpretation of tactile signals. 

Illusion: something that deceives by producing a false or misleading impression 

of reality. 

Thaumatrope: a card with different pictures on opposite sides, such as a horse on one side and a 

rider on the other, which appear as if combined when the card is twirled rapidly, thus illustrating 

the persistence of visual impressions. 

Stroboscope: a device for studying the motion of a body, especially a body in rapid revolution or 

vibration, by making the motion appear to slow down or stop, as by periodically illuminating the 

body or viewing it through widely spaced openings in a revolving disk. 

References 

»» 

Listverse | Top 10 Incredible Sound Illusions 

http://listverse.com/2008/02/29/top-10-incredible-sound-illusions/ 

»» 

Accoustical Society of America | The Speech-to-Song Illusion 

http://www.acoustics.org/press/156th/deutsch.html 

»» 

Wikipedia | Sensory receptor 

http://en.wikipedia.org/wiki/Sensory_receptor 

»» 

Wikipedia | Lateral inhibition 

http://en.wikipedia.org/wiki/Lateral_inhibition 

»» 

Wikipedia | Hermann grid 

http://en.wikipedia.org/wiki/File:HermannGrid.gif 

»» 

Magic Eye | Frequently Asked Questions 

http://www.magiceye.com/faq.htm 

»» 

Vern’s SIRDS Gallery 

http://www.vern.com/ 

»» 

SIRDS | List of Random Dot Stereogram Websites 

http://www.nottingham.ac.uk/~etzpc/sirds.html 

»» 

Interactive Mathematics Miscellany and Puzzles | Single Image Stereograms 

http://www.cut-the-knot.org/Curriculum/Geometry/Stereo.shtml 

»» 

Gary Beene’s Stereograms Information Center | How to Create Stereograms 

http://www.garybeene.com/stereo/rds-over.htm 


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»» 

Wikipedia | Necker Cube 

http://en.wikipedia.org/wiki/File:Necker_cube.svg 

»» 

Wapedia | Wiki | Necker Cube 

http://wapedia.mobi/en/Necker_cube 

»» 

Wise Bread | Optical Illusions That Make You Fatter and Your Wallet Lighter | Circle size 

http://www.wisebread.com/optical-illusions-that-make-you-fatter-and-your-wallet-lighter 

»» 

Braingle | Brainteasers | Old or Young Woman 

http://www.braingle.com/wii/brainteasers/teaser.phpid=26745 

»» 

Squidoo | Optical Illusions Images | Duck or Rabbit 

http://www.squidoo.com/optical-illusion-images 

»» 

Dartmouth Computer Science | Hany Farid | A Collection of Visual Illusions 

http://www.cs.dartmouth.edu/farid/Hany_Farid/Illusions/Illusions.html 

»» 

Wikipedia | Thaumatrope 

http://en.wikipedia.org/wiki/Thaumatrope 

Other resources 

»» 

Thomas FX Group Inc. | Rubber Hand (to purchase) 

http://www.thomasfx.com/ 

NOTES 


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GAME 

Materials 

• small container (box, 

bucket, bin) 

• image templates 

ACTIVITY 1: Image decoder - 20mins 

This relay-race/puzzle game will reinforce the similar, yet unique, functions of the rods 

and cones in our eyes. Images are formed by the combination of information received 

by rods and cones on the retina. Rods are better at discriminating between different 

brightness levels (light vs dark), while cones are better at perceiving different colours. 

What to Do 

Preparation: 

1. Print out templates (one colour, one black and white). 

2. Cut each image into 15 puzzle pieces. Laminate the pieces if desired. 

Instructions: 

1. Gather students at one end of an open space. 

2. Put the puzzle pieces into the container and place the container at the other end of the 

play area. 

3. Gather the class in a circle. Assign students to a team (rod or cones) by alternating their 

placement. 

4. Starting in one place on the circle, one member of each team will run to the container 

and pick up an image piece. If they’re on Team Rods, they should pick up a black & 

white puzzle piece; if they’re on Team Cones, they should pick up a colour puzzle piece. 

Students then run back to their place in the circle. 

5. Once a team has collected a few puzzle pieces, they can begin assembling their image. 

The completed image will contain all 15 pieces. 

6. The first team to assemble their image correctly wins. 

Key Questions 

»» 

Did rods and cones see the same image 

»» 

What was different about the image 

Extensions 

»» 

This activity can be run either as an outdoor game, or as a classroom activity 

without the relay-race component. 


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DEMO 

ACTIVITY 2: Do as I say, not as i do! - 5mins 

This demonstration shows the importance of visual cues. 

If we get conflicting information from our eyes and ears, many of us will default to the 

information we receive from our eyes. Most of us rely on vision as our dominant sense 

to understand the world around us because we can get so much information this way: 

shape, size, distance, motion, and colour. 

In this demonstration, the information from our eyes is different from the information 

from our ears. Most of us “listen” to our eyes in this situation. 

What to Do 

Instructions 

1. Lead the students through the following instructions, while performing the 

actions yourself: 

Stick out your hand. 

Now stick out your thumb. 

Now stick out your forefinger. 

Touch them together to make a ring. 

Now put the ring on your chin. [said while placing your ring on your cheek]. 

How many of you are touching your CHIN” 

Key Questions 

»» 

Why are you touching your cheek 

»» 

Which sense took over: sight or hearing 

Extensions 

»» 

Create your own visual cue illusion and try it on family and friends! 


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DEMO 

Materials 

• Hermann Grid template, printed 

on either an overhead sheet 

or on a large sheet of paper 

overhead projector (optional) 

ACTIVITY 3: SEEING SPOTS - 5mins 

In this demonstration, students see imaginary spots and relate this illusion to the 

structure of the retina. 

The Hermann Grid is an illusion based on the way our eyes work. Most people "see" grey 

spots where lines of white space intersect, even though there is no grey there at all. 

This illusion is an example of lateral inhibition — a feature of our visual system. 

Light sensitive cells, or rods, are arranged in rows on the retina. It is possible that light 

hits just one rod, Rod X, which sends a signal to the brain. But if light also hits Rod X's 

neighbours, Rod X's signal won't be as strong. It’s as though the sensitivity of Rod X 

is ‘turned down’ when its neighbours also receive light. 

In Hermann's Grid, the places where the white lines cross have white surroundings 

in four different directions. The rods getting light from the middle of the intersection 

have neighbour cells in every direction that also get lots of light. So the middle rods are 

‘turned down’ and the intersection looks darker. 

These spots don’t generally appear directly where you’re looking but rather a little off 

to the side. This is due to the fact that there are no rods cells directly in the centre of the 

retina. It also explains why dim stars tend to ‘disappear’ when we look straight at them 

and look brighter when we look slightly beside them. 

Key Questions 

»» 

What do you see 

»» 

Are the gray dots really there 

»» 

Why do you think they appear 

NOTES 


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DEMO 

Materials 

• a selection of single image 

random dot stereograms, 

printed on either an overhead 

sheet or on a large sheet of 

paper 

http://www.nottingham. 

ac.uk/~etzpc/sirds.html 

• overhead projector (optional) 

ACTIVITY 4: Random Dot Stereograms - 30mins 

In this demonstration, students discover how the structure and placement of the eyes 

creates depth perception. 

Depth perception occurs as your brain compares the pictures received in each eye to 

figure out how far objects are from you. Each eye sees a slightly different picture because 

they’re in slightly different places. 

If you use first one eye then the other to view an object, nearby objects tend to jump back 

and forth more, whereas faraway objects hardly seem to move at all. 

In the first part of the demonstration, one eye sees the object covered by the index 

finger. The other eye has an unobstructed view. This demonstrates that each eye is 

getting a different picture of the world. 

Single image random dot stereograms are the original versions of the popular ’Magic 

Eye’ pictures in which a 3D image pops out from what appears to be a sheet of random 

dots. The dots are arranged in repeating pattern, with slight differences in each 

repetition. Each eye sees a slightly different pattern because of the different angles 

between the page and each eye. Your brain tries to overlap the two patterns, and creates 

the virtual 3D object. 

If you want to view movies or pictures in 3D, you have to show a different picture to each 

eye. That’s why they give you glasses with 2 lenses of different colours (or let through 

light of different polarities.) 

What to Do 

Instructions 

Part 1: Depth Perception 

1. Focus on a person or object. 

2. Hold up your index finger at arm’s length. 

3. Close your left eye. Looking only with your right eye, move your finger so it covers the 

object. 

4. Now look with just your left eye. 

Part 2: Stereograms 

1. Show random dot stereograms to students. 

2. Try and see what image is “hidden” in the stereograms. 

Activity 4 cont. 


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Key Questions 

»» 

Do both eyes have the same point of view of the objects before them 

»» 

If you bring your finger closer to your face, do you find it jumps back and forth more or 

less than when it’s farther away 

»» 

What was your method for “seeing” the image 

Extensions 

»» 

How would being blind in one eye affect your depth perception Why 

»» 

How are random dot stereograms created 

NOTES 


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DEMO 

Materials 

• Wire Necker Cube template, 



paper for the entire class to 

see. 


ACTIVITY 5: NECKER cUBE - 5mins 

In this demonstration, students are challenged to determine the orientation of the cube, 

but their brains have a difficult time making a decision. 

We use lines in objects for clues about depth. In the Wire Necker Cube, it is not clear 

which of the two vertical lines in the middle of the cube should be closer to you. Usually, 

we assume the longer line is the closer one and the shorter one is further away, but in 

this illusion the two lines are the same length. 

What to Do 

Instructions 

1. Show the Wire Necker Cube to the class. 

2. Ask whether the cube is popping out of the page or going into the page. 

Key Questions 

»» 

Is it difficult to decide which way the cube is going 

»» 

Why do you think it is so hard to decide 

»» 

What cues does our brain normally use to determine the orientation of an object 

Extensions 

»» 

Challenge students to draw their own Wire Necker Cube. 

NOTES 


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DEMO 

Materials 

• circles template, printed on 

either an overhead sheet or on 

a large sheet of paper overhead 

projector (optional) 

• 2 loonies or toonies 

ACTIVITY 6: SIZE PERCEPTION - 5mins 

In this demonstration, students learn about the importance of context in our brain’s 

ability to interpret information. 

In this illusion, one circle seems small compared to its large neighbours and the other 

seems large compared to its small neighbours. This happens because we hardly ever 

look at objects by themselves: our brains automatically seek to compare them to 

surrounding objects in order to determine their size. When a coin is placed in the middle 

of each circle, the brain is able to directly compare their sizes, dissolving the illusion. 

This illusion can be used to the benefit of people who want to reduce their portion size 

when eating. Eating on a small dinner plate gives the illusion that we are eating more 

food than if it is served on a large dinner plate. 

What to Do 

Instructions 

1. Show the students the circle image. 

2. Borrow two large coins (loonies or toonies) and hold them in the middle of each circle. 

Key Questions 

»» 

Which middle circle is larger 

»» 

What made you think that the circles were different sizes 

Extensions 

»» 

Challenge students to create their own size illusion. 

»» 

How could this illusion help a person who is trying to control their portion size 

NOTES 


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DEMO 

Materials 

• Old/Young Woman template, 



paper 

• Duck to Rabbit template, 



paper for the entire class to 

see. 


ACTIVITY 7: What do you see - 5mins 

In this demonstration, students may argue as they try to make sense of an ambiguous 

picture with two very different interpretations. 

This illusion works because the image is ambiguous, which means there is just enough 

information for you to make sense of it in a couple of different ways. Our brain is usually 

able to make sense of the information it receives. In this case, however, there are two 

reasonable interpretations that can be made. 

What to Do 

Instructions 

1. Show the students the Old/Young Woman image. Ask them whether they see a young 

woman or an old woman. 

2. Show the students the Duck to Rabbit image. Ask how many people see the rabbit. 

Rotate the illusion so that the rabbit ears are pointing up. 

Key Questions 

»» 

How old is the woman in the picture 

»» 

Who sees a young woman An old woman 

»» 

Why do you think we can see two different things 

»» 

If you concentrate on certain parts of the image, your brain will make sense of it in 

different ways. What parts help you see the old woman The young woman 

»» 

How is the Duck to Rabbit image similar/different from the Old/Young Woman image 

»» 

Why does the animal you see change when you rotate the image 

NOTES 


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DEMO 

Materials 

• Stroop Effect template, printed 

on either an overhead sheet or 

on a large sheet of paper 


• stopwatch (optional) 

ACTIVITY 8: stroop effect - 10mins 

In this fun demonstration, students try and consciously change the interpretation the 

brain naturally wants to give them. 

Our experiences in the past help us look for patterns and make assumptions in the 

present. This demonstration is known as the Stroop Effect, named after its discoverer, 

J. Ridley Stroop. Your natural tendency is to read words, not report the colour of the ink 

in which the word is written, which is why it’s difficult for you to say the colour instead 

of the word itself. Even if the Stroop words were upside down, the brain would still 

recognize the shapes as letters and group them together as words to read. 

Teacher’s Note: This image MUST be printed in colour. 

What to Do 

Instructions 

1. Challenge the class to read the names of the colours out loud together. This should be 

fairly easy. 

2. Challenge the students to name the colour that the words are written in. For example, if 

the word “green” is written in purple, the class would name the colour “purple”. 

3. If using stations, have students use a stopwatch to time themselves reading the words 

versus naming the colours and compare the results. 

Key Questions 

»» 

Why do you think it is so difficult to report the colour the word is written in 

Extensions 

»» 

Turn the words upside down and name the colours. Does this change the results Why 

NOTES 


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blue purple green red 

orange blue red green 

purple orange red blue 

green red blue purple 

orange blue red green 

green purple orange red 


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DEMO 

Materials 

• green Canadian flag template, 



paper 


• large white board, wall or card 

ACTIVITY 9: Canadian flag after-image - 5mins 

In this demonstration, colours appear before the students’ eyes as they learn about cell 

fatigue and afterimages. 

Our central nervous system stops responding to information from our senses that is 

constantly repeated. You’re not aware of the feeling of your clothes on your skin (until 

I mentioned it) because that feeling doesn’t change. This is because you have adapted 

to the stimulus (the feeling of your clothes). This is useful so that you will notice a 

sudden and threatening change in your environment, such as a tiger attack. It would be 

unfortunate if your brain was busy paying attention to your clothes touching your skin 

and didn’t notice a tiger! 

Staring at the green colour in the flag tires out the cells in your retina that respond to green. 

Since the brain gets used to looking at the green in the flag, the cone cells send fewer and 

fewer signals saying “green stuff here!” to the brain. While you’re looking at the flag, your 

eyes are technically less sensitive to green since it is “old news” for the brain. 

When you switch over to a white surface, your eyes don’t register the green light and 

you see its complementary color red for a few moments. Remember that what our brain 

processes as “white” is actually the combination of all colours combined. It takes a few 

moments for the cones to start firing green signals to the brain again, which is why we 

see the complementary color red. 

In the same way, if you stare at a red color for 30 seconds or more, the cells in your 

retina that respond to red will fatigue more and fire less. When you switch over to a white 

surface, your eyes subtract the red and you see its complementary color green. 

Teacher’s Note: This image MUST be printed in colour. 

What to Do 

Instructions 

1. Hold up the peculiar version of the Canadian flag. 

2. Instruct students to stare at the image for 30 seconds (you can hum the National Anthem 

while they stare). 

3. Take the green flag away, and tell the students to stare at a white surface. 

Key Questions 

»» 

What do you see when you stare at the white wall 

»» 

Why don’t we see a green maple leaf, like the original flag we looked at 

Extensions 

»» 

Try making your own after-image. Use different colours and record the complementary 

colour that appears. 


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Illusions 


make & take 

Materials 

Per student: 

• Benham’s disk template 

(printed on cardstock) 

• a pin 

• a pencil with an eraser tip 

• scissors 

ACTIVITY 10: benham's disk - 30mins 

Students make a contraption that appears to produce colours from black and white. 

Benham's Disk was invented by a nineteenth-century toymaker who noticed colours in a 

black-and-white pattern he had mounted on a spinning top. 

Why do we see colours There are three types of cones. One is most sensitive to red 

light, one to green light, and one to blue light. Each type of cone has a different latency 

time, the time it takes to respond to a stimulus, and a different persistence of response 

time, the time it keeps responding after the stimulus has been removed. Blue cones, for 

example, are the slowest to respond and keep responding the longest. 

When you gaze at one place on the spinning disk, you are looking at alternating flashes 

of black and white. When a white flash goes by, all three types of cones respond. Your 

eyes and brain see the colour white only when all three types of cones are responding 

equally. The fact that some types of cones respond more quickly than others and that 

some types of cones keep responding longer than others leads to an imbalance that 

partially explains why you see colours. 

The colours vary across the disk because the black arcs have different lengths, so that 

the flashing rate they produce on the retina is also different. 

What to Do 

Instructions 

1. Cut out the Benham’s disk template. 

2. Poke a pin through the middle of the circle and then fix the disk to the pencil by poking 

the pin into the eraser. 

3. Spin the disk and observe what happens. 

Key Questions 

»» 

What do you see What changes when you start spinning the disk 

»» 

Where do you think the colours come from 

»» 

Why are there different colours in different places on the disk 

Extensions 

»» 

Try spinning the disk in the opposite direction. 

»» 

Try spinning the disk in dim light. 

»» 

Full Lesson | Colour 

http://resources.scienceworld.ca/lessons-by-topic/light/1141-colour.html 


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make & take 

Materials 

Per student: 

• bird and branch template 

• straw 

• scissors 

• 2 pieces of tape 

ACTIVITY 11: Thaumatrope - 30mins 

Students take advantage of persistence of vision to combine two separate images. 

This illusion takes advantage of something called “persistence of vision”. When an 

image is shown to your eyes, the retina keeps responding for a short time (about 1/30th 

of a second) after the image itself has gone away. If a new picture appears just before the 

previous one has faded away, your brain blends the two together. 

A thaumatrope is a toy, popular in Victorian times, that uses this persistence of vision. 

What to Do 

Instructions 

1. Cut out the circles containing the bird and the branch. 

2. Tape down the straw to the backside of one of the images. 

3. Tape the backs of the 2 images together. 

4. Spin the straw between your hands. The bird should appear on the branch. 

Key Questions 

»» 

What happens when you twirl the straw 

Extensions 

»» 

Design your own thaumatrope (e.g. fish in a bowl, spider on a web etc). The only tricky 

part is positioning the 2 images so that they line up correctly when you twirl them. 

»» 

How is this optical illusion similar to the Canadian Flag After-Image 


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make & take 

Materials 

• large mirror for students to 

view their stroboscopes in 

motion 

Per student: 

• scissors 

• stroboscope template (printed 

on light cardboard) 

• pin 

• pencil with eraser at one end 

ACTIVITY 12: STROBOSCOPE - 30mins 

Students take advantage of persistence of vision with a simple apparatus that turns a 

series of static images into motion. 

In a stroboscope, slits open and close in front of your eye as the disc rotates. Each time a 

slit opens, you glimpse the scene on the far side of the disk. Each open-slit image lingers 

in your eye and brain long enough to merge with the next image. This phenomenon, 

called “persistence of vision”, will give your brain the illusion that motion is taking place. 

What to Do 

Instructions 

1. Cut out the stroboscope template. Don’t forget to cut out the slits around the edge! 

2. Poke a pin through the middle of the circle then fix the disk to the pencil by poking the 

pin into the eraser. 

3. Spin the template and look through the slits at the back towards a mirror. 

Key Questions 

»» 

What do you see in the mirror 

»» 

Why does it seem that the image is moving 

Extensions 

»» 

On the reverse side of the stroboscope you have cut out, create your own changing image 

using either dots (to represent a ball dropping), or your own drawing. Start off with 

something simple and then see if you can make it even more challenging! 


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DEMO 

ACTIVITY 13: P-O-T-S - 5mins 

Students discover how auditory illusions are based on confounding assumptions 

and patterns. 

After repeating P-O-T-S several times, most people will say “Stop” because their brains 

have been “primed” by the letters P-O-T-S. The human brain is so good at finding 

patterns that it recognizes POTS as being the reverse spelling of STOP and relates it to 

our behaviour at a traffic light, all before our conscious mind can catch up. 

What to Do 

Instructions 

1. Ask the class to spell the word "POTS" along with you. This works best as a call and 

response: 

Instructor: P-O-T-S Class: P-O-T-S 

Instructor: P-O-T-S Class: P-O-T-S 

(repeat 5/6 times) 

2. Immediately ask the class: "What do you do at a green light" 

Key Questions 

»» 

If you know the answer to what you do at a green light, why do you think you answered 

the question incorrectly 

Extensions 

»» 

Create your own auditory illusion and test it on other students or your family 


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DEMO 

Materials 

• computer or mobile device with 

speakers (or headphones if 

using as a station) 

• audio link to the Diana 

Deutsch’s Speech-to-Song 

Illusion 

http://www.acoustics.org/ 

press/156th/deutsch.html 

ACTIVITY 14: Speech-to-Song Illusion - 10mins 

Students listen to an auditory illusion that blurs the line between speech and song. 

This is an illusion discovered by Diana Deutsch at the University of California, San 

Diego. Some researchers spend their entire academic careers studying how the brain 

misinterprets information, leading to illusions. 

Exactly why this illusion occurs is not fully understood, but it does suggest a strong link 

between speech and music. In Deutch’s study, even professional vocalists with years 

of musical training and a well-tuned ear hear a melody where no melody exists. Her 

observations tell us that although the ear may hear the same words with no variation, 

the brain can transform the information into more than one result, such as spoken words 

or a melody. 

What to Do 

Instructions 

1. Ask students to close their eyes. 

2. Play the clip “Sound Demo 1”. 

3. Ask the class to repeat/sing the phrase they heard. 

4. If anyone in the class only heard the phrase as spoken, and not sung, either play “Sound 

Demo 1” again, or play “Sound Demo 3”. 

5. When everyone can hear the phrase as sung, sing the phrase as a class again. 

6. Play “Sound Demo 2”. 

7. Discuss what they heard. 

Key Questions 

»» 

How does the statement “sometimes behave so strangely” change from the beginning to 

the end of the demo 

»» 

At what point did the spoken phrase appear to transform into song 

Extension 

»» 

What makes a song a song Can you describe the difference between words spoken 

and sung 

»» 

Try other amazing auditory illusions | Top 10 Incredible Sound Illusions 

http://listverse.com/2008/02/29/top-10-incredible-sound-illusions/ 


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ACTIVITY 

ACTIVITY 15: Numb Fingers - 5mins 

In this activity, students confuse their brains as they explore a phenomenon known as 

perceptual disjunction. 

Sometimes our brains fail to take all factors into account when processing sensory 

information. The sensation we think we feel is therefore incomplete. For example, in the 

activity below, our brains “forget” that there is another object (your partner’s finger) 

blocking us from feeling both sides of our index finger. Stroking only one side of our 

finger results in the feeling that our finger has gone numb. 

What to Do 

Instructions 

1. Pair up with a partner. 

2. Place the palm of your left hand against the palm of your partner’s right hand, making 

sure that all the fingers line up. 

3. Use the thumb and index finger on your right (other) hand to simultaneously stroke your 

and your partner’s index fingers (i.e. your right thumb should be touching your index 

finger, and your right index finger should be stroking your partner’s left index finger). 

4. Swap roles with your partner. 

Key Questions 

»» 

How does your finger feel 

»» 

How are you confusing your brain 

Extensions 

»» 

How is this illusion similar to optical illusions 


Illusions 


ACTIVITY 

Materials 

Per pair of students: 

• paper clip 

ACTIVITY 16: finger-to-elbow - 10mins 

In this activity, students discover how the physical distribution of sensory receptors in 

our bodies can create a tactile illusion. 

Sensory receptors are not distributed evenly across our bodies. Some parts of our body, 

like our fingertips, are very densely packed with nerve endings and touch receptors. 

They are highly sensitive to touch, and can differentiate between even very fine, minute 

touch sensations. Other parts of our body, like our forearms, are less packed with touch 

receptors, so they are not as good at sensory discrimination. If you move from an area of 

higher sensory discrimination to a lower one, you can experience something very odd– 

two distinct sensations can suddenly feel like just one! 

What to Do 

Part 1: Finger 

1. Pair up with a partner and sit facing each other. 

2. Have your partner hold out their left arm and close their eyes. 

3. Lightly place your finger on their index fingertip, and SLOWLY move your finger up their 

forearm towards their elbow (keep your finger on the inside of their forearm and move 

towards the ‘crook’ of their elbow). 

4. Let your partner know to tell you when they think you’ve reached their elbow. They can 

open their eyes to see how accurate they were! 


Part 2: Paper Clip 

1. Unfold a paper clip into 1 straight line, then bend it in half so it looks like a horseshoe, 

with the clip-tips about 1cm apart. 

2. Repeat the activity of part one, but gently use the 2 tips of the paper clip instead of your 

finger. 

3. Have your partner tell you when they no longer feel 2 distinct clip-tips, but 1. 


Key Questions 

»» 

Why does it feel like your partner has reached your elbow, when they haven’t 

»» 

Where can you tell there are 2 clip-tips, and where can you only distinguish one 

»» 

Which part(s) of your arm have more sensory receptors How do you know 

Extensions 

»» 

Try parts 1 and 2 going the opposite way (i.e. from crook-of-the-elbow to fingertip). 


Illusions 


ACTIVITY 

Materials 

• rubber fake hand (child-size 

rubber hand, if possible 

– available in Halloween/ 

costume stores) 

• 2 paint brushes 

• 2 black cloths (big enough 

to cover a student’s entire 

forearm) 

• standing barrier (e.g. hardcovered 

book) 

• hammer (optional) 

ACTIVITY 17: fake hand - 10mins 

Which is our dominant sense If we get conflicting signals from different senses, which 

one do we tend to believe Students investigate the answers to these questions in the 

following activity. 

Can you touch your finger to your chin with your eyes closed Our brain has an internal 

“map” of our entire body that allows us to do things like this. Even without the visual 

guidance of our sense of sight, our fingers still know where to go. Our sense of the 

relative positioning of the different parts of our body is referred to as proprioception. 

This internal map is so ingrained in us, that people who have lost limbs due to accident 

or amputation will often report still being able to feel sensations from their (missing) 

limb–a phenomenon known as the Phantom Limb Syndrome. Many people will continue 

to experience this sensation for months after their accident, until their brain develops a 

new map of their body. 

Our proprioception can also be manipulated the other way around, such that we come 

to think that something that looks similar to one of our body parts actually IS one of our 

body parts. When we receive conflicting inputs from different senses, we tend to believe 

what our eyes are telling us above all else, as vision is the dominant sense in humans. 

In this activity, when we see a fake hand being stroked while also feeling our real-buthidden 

hand being stroked, we can be induced into believing that our real hand has 

somehow been transposed into the fake one. 

Teacher’s Tip: This activity is best performed as a station so that all of the students get a 

chance to give it a try. 

Fun Fact: 

»» 

Anecdotal evidence from Science World testing seems to indicate that males are more 

susceptible to this illusion than females. 

What to Do 

1. Have a volunteer sit at the table, with their left hand resting on the table a little off 

to the side. 

2. Cover their entire forearm with one black cloth, leaving only their hand and 

wrist exposed. 

3. Place the fake rubber hand directly in front of the volunteer, and cover it with a second 

black cloth, exposing the same amount of wrist/hand surface as the volunteer’s 

own hand. 

4. Set up the barrier between the fake hand and the real one, such that the volunteer 

cannot see their real hand. 

Activity 17 cont. 


Illusions 


5. Take two paintbrushes and stroke the fake hand and real hand simultaneously (i.e. 1 

paintbrush for each hand), in long slow strokes from wrist to fingertip. It is CRUCIAL to 

have the exact same brush placement on both hands: if you are stroking the index finger 

on the fake hand, be sure you are also stroking the index finger on the volunteer’s real 

hand. 

6. After approximately 1 minute of stroking, take out the hammer (or use your fist) and 

pound hard on the fake hand. See if your volunteer jumps! 

Key Questions 

»» 

Why did you jump, even though you knew it wasn’t your hand that was hit 

»» 

In this activity we got conflicting signals from different senses. Which sense 

did you believe 


Illusions 


ACTIVITY 

Materials 

Per pair of students: 

• a pencil 

• a plastic comb (flat comb 

with flexible bristles) 

ACTIVITY 18: comb dot - 10mins 

In this illusion, students take advantage of a (typically) unfamiliar sensation for our 

fingertips’ touch receptors: having the skin on our fingertips stretched. 

When we place our fingertips against a brush-like surface with semi-rigid bristles, and 

then move the bristles back and forth, the skin on our fingertips becomes alternately 

stretched and compressed by the undulating motions. Because of the novelty of this 

particular sensation, our touch receptors have difficulty interpreting it properly. Instead, 

we reinterpret the stretch/compress motion as the more familiar sensation of running 

our finger across a raised surface, like a dot. 

What to Do 

Instructions 

1. Pair up with a partner. 

2. Take the comb and hold it sideways (as it would naturally lie on a flat surface). 

3. Hold the comb in one hand, and place the index finger of your other hand on top of a 

section of bristles. 

4. Have your partner hold the pencil vertically–perpendicular to the horizontal comb–and 

run the pencil continuously along the bristles of the comb horizontally. 


Key Questions 

»» 

What do you feel 

»» 

Do you feel the bristles move from side to side 

»» 

Why do you think we feel something different to what is actually happening 

»» 

In this case, which sense has taken over: sight or touch

PERCEPTION AND ILLUSIONS - Science World Resources

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