School Science Lessons
Biology
2025-09-26
Optical illusions
Contents
9.0 Optical illusions
9.1 Absent square illusion
9.2 Afterimages illusion
9.3 Barberpole illusion
9.4 Bending pencil illusion
9.5 Bent stick illusion
9.6 Blivet illusion
9.7 Cloud shapes illusion
9.8 Count the boxes illusion
9.9 Duck-rabbit illusion
9.10 Ebbinghaus illusion
9.11 Feel two noses
9.12 Fraser spiral illusion
9.13 Ghost coin illusion
9.14 Hermann Grid illusion
9.15 Impossible square illusion
9.16 Impossible triangle illusion
9.17 See: Mirror images (inversion, lateral inversion)
9.18 Man in the Moon illusion
9.19 Mirage illusion
9.20 Moving window illusion
9.21 Müller-Lyer illusion
9.22 Necker cube illusion
9.23 Poggendorf illusion
9.24 Ponzo figure illusion
9.25 Raindrops illusion
9.26 Refraction in water illusion
9.27 Retention of image illusion
9.28 Reversible figures illusion, Rubin vase illusion
9.29 Rising coin illusion
9.30 Rising moon illusion
9.31 Sense of touch
9.32 Size-weight illusion
9.33 Slice of pie illusion
9.34 Spinning picture illusion
9.35 Spoon in water illusion
9.36 Sun and Moon diameter illusion
9.37 Vertical-horizontal illusion
9.38 Young girl-old woman illusion
9.0 Optical illusions
"Magic Wire Illusion", (toy product)
"Illusion Science Pack", (toy product), over 20 optical illusion activities
Optical illusions caused by disturbance of light between objects and the eyes are different from illusions caused by the disturbance of sensory signals of the eye.
Cognitive illusions are caused by misapplied knowledge employed by the brain to interpret or read sensory signals are quite different.
Prof. Richard Langton Gregory (1923-2010), England, has classified optical illusions as follows:
1. Ambiguity illusions
Optical disturbance intervening between the object and the retina.
See diagram 9.246.2: Necker cube illusion.
2. Distortion illusions
Disturbed neural sensory signals, mirages, perspective problems, human eye astigmatism
See diagram 28.122.2: Bent stick illusion.
See diagram 28.127.2: Spoon in water illusion.
See diagram 28.122.3: Rising coin illusion.
See diagram 28.122.1: Depths in a swimming pool.
3. Paradox illusions
How to make sense of neural signals.
See diagram 9.247.5: Impossible triangle, Penrose triangle
4. Fiction illusions
9.7: Cloud shapes illusion
9.1 Absent square illusion, Kanizsa triangle illusion
Absent square illusion, Kanizsa triangle illusion, square that is not there illusion
See diagram 28.1.1.7: Absent square illusion.
In the Absent square illusion and Kanizsa triangle illusion the brain creates illusory contours.
The human brain tries to make sense of all sense perceptions, even to "seeing" a square that is not there!
Some illusions are caused by the brain "'postulating" a nearer occluding surface, to "explain" surprising gaps.
A cut-out square in black paper has the illusion of being a white square on top of black paper.
9.2 Afterimages illusion
Afterimages illusions are caused by local loss of retinal visual pigments following intense or prolonged stimulation.
The three types of colour receptor cones are most sensitive to red, blue or green.
The cones get fatigued when you stare at a particular colour.
Then when you look at the white background, the colour receptors are not in balance and you see the colour "afterimages."
9.3 Barberpole illusion
See diagram 9.247.8: Barberpole illusion.
A diagonally-striped pole is turned around its vertical axis.
The stripes appears to move up or down.
The original custom was for barbers to dry the bandages used in blood letting by wrapping them around a pole.
9.4 Bending pencil illusion
Hold one end of a pencil between the thumb and forefinger.
Extend the arm, hold the pencil loosely and parallel to the eyes.
Shake the pencil up and down very vigorously by a vertical distance of five cm.
The pencil appears to bend.
This illusion may be caused by persistence of vision.
Perhaps the eye movements of the observer do not track the motions of parts of the shaken object.
9.5 Bent stick illusion
See diagram 9.122.2: Bent stick illusion.
9.6 Blivet illusion
See diagram 9.246.9: Blivet, devil's fork.
The word "blivet" has different meanings, but here it means an impossible figure.
The brain switches perception of a two-pronged fork or three-pronged fork.
In this series of optical illusions is where an artist does a perspective drawing, and crosses over some of the lines at one end to get a pseudo view as well.
Usually they start with a drawing of a tuning fork, as in the devil's fork.
For example: See diagram 9.255: Four or three bars?
9.7 Cloud shapes illusion
Hamlet by William Shakespeare, Act 3 Scene 2
"Hamlet: Do you see yonder cloud that's almost in shape of a camel?
Polonius: By th' mass, and 'tis like a camel indeed.
Hamlet: Methinks it is like a weasel.
Polonius: It is backed like a weasel.
Hamlet: Or like a whale.
Polonius: Very like a whale.
Hamlet: Then I will come to my mother by and by.
They fool me to the top of my bent. - I will come by and by."
9.8 Count the boxes illusion
See diagram 9.246.6: Count the boxes.
The same drawing of a pile of boxes seen inverted changes the number of boxes.
9.9 Duck-rabbit illusion
See diagram 9.247.4: Duck-rabbit.
The brain keeps switching from perception of a duck to perception of a rabbit.
9.10 Ebbinghaus illusion
See diagram 9.248.1: Ebbinghaus illusion.
Hermann Ebbinghaus, Germany (1850–1909).
An optical illusion of relative size perception.
The central circle surrounded by large circles appears smaller than the central circle surrounded by small circles, but the two central circles are the same size.
9.11 Feel two noses
Tactile illusion
Cross the middle finger over the index finger.
Close your eyes.
Move the tips of these fingers along your nose with one finger each side of the nose.
When your fingers reach the tip of your nose your fingers feel further apart and you get the illusion (Aristotle's illusion) that you have two noses, because your brain assumes that your finger tips are in the usual position.
You can get the same illusion by holding a marble, pencil, or another person's finger, (dead man's finger). between the crossed fingertips.
9.12 Fraser spiral illusion
Fraser spiral, false spiral, twisted cord illusion
See diagram 9.246.4: Fraser spiral illusion.
Apparent spiral.
Overlapping arcs appear to form a spiral, but the arcs are concentric circles.
9.13 Ghost coin illusion
See diagram 9.246.3: Ghost coin.
Grip two coins between the tips of the index fingers held vertically.
Rub the coins against each other with rapid up and down movements.
A third ghost coin appears between and below the real coins.
The retina retains images on the real coins in their lower position.
9.14 Hermann Grid illusion
See diagram 9.246.5: Hermann Grid illusion.
Ludimar Hermann, Germany, (1833-1914)
Dark patches appear in the street crossings, except the ones which you are directly looking at.
Look at the 16 black squares in the diagram and note that the white bars between the black squares appear white
then where the white bars intersect where dark areas appear on the intersections.
However, if you look closely at the intersections and not at the black squares the dark area disappear.
White contrasted with black appears whiter when we use your peripheral vision.
Look at the black and white discs in the squares.
The white discs appear larger.
White objects appear larger when contrasted with a black background.
9.15 Impossible square illusion
See diagram 9.247.5: Impossible square, Impossible triangle.
The surfaces and corners of the square are not in the same position as in a normal square.
Try to draw an impossible square!
9.16 Impossible triangle illusion
Impossible triangle, Penrose triangle, Penrose tri-bar illusion
See diagram 9.247.5: Impossible triangle, Impossible square
An "impossible triangle" can be constructed, so that by observing the construction of it from a certain angle it gives the impression of being an impossible figure.
The lithograph print "Belvedere" by M. C. Escher, 1958, Holland, shows a building that is an impossible object.
See diagram: "Belvedere".
9.18 Man in the Moon illusion
1. Observe the craters and flatter areas, "seas" (Mare), and oceans (Oceanus).
The space craft Apollo 12 was launched 14 November 1969 and landed on the Oceanus Procellarum on 19 November 1969.
Then the astronauts walked to the remains of previous lunar probe Surveyor 3 and retrieved some pieces of it.
The arrangement of craters, sea oceans and other features allow different people and cultures to see figures in the Moon.
Although "the man in the Moon" in the Northern Hemisphere looks like an old man walking away carrying sticks or leaning on a fork, some people can see different faces and figures, even a frog.
In China and Japan they see a large rabbit stretched across the Moon with the ears pointing down from the upper right and the legs crossed at the lower left.
The rabbit is making something in a box.
Most figures can be seen only at or near a full Moon.
Some of these figures appear differently in the Northern and Southern Hemisphere.
Stare at the Moon at different phases until you can see figures.
Record the figures on a Moon diagram and note the time and date of the observations.
2. Measure the angular diameter (visual angle), of the Moon, about 29.3 ' to 34.1'.
Its actual diameter is about 3476 km (equatorial), and about 3472 km, (polar).
9.19 Mirage illusion
A mirage is an optical illusion usually caused by refraction in heated air.
It may also appear as the reflected image in the sky of a distant object.
A mirage results from seeing through turbulent air For example, puddles of water may appear on an ascending hot road, a water lake may appear in the desert, heat shimmers over engines.
A mirage can be photographed, so it is not an optical delusion!
Experiments
1. Look at an object on the other side of a hot engine or a hot road.
The object will appear distorted, because the refractive indexes of warm and cold air are different.
This is one cause of mirages in the desert.
2. To create a mirage, direct the image from a slide projector just above a brass plate heated with a Bunsen burner.
9.20 Moving window illusion
When observing objects and features of the countryside from the window of a moving vehicle, closer objects and features appear to move faster than those further away.
Further object have a smaller angular velocity than closer objects, so they take longer to pass through the field of vision.
Look at text between the V formed by two stretched fingers of your hand flat on the page.
Move your hand across the page and observe the letters closer to the hand within the V appearing to move faster than the letters between the fingertips.
9.21 Müller-Lyer illusion
See diagram 9.247.2: Müller-Lyer illusion.
The shaft of the outgoing arrow-heads appears longer than the shaft for the ingoing heads.
9.22 Necker cube illusion
See diagram 9.246.2: Necker cube illusion.
In this line drawing do you see the lower left square in front or the upper right square in front?
9.23 Poggendorff illusion
See diagram 9.248.3: Poggendorff illusion.
Johann Christian Poggendorff, Germany, (1796 - 1877)
A straight line crossing a rectangle appears displaced.
9.24 Ponzo figure illusion
See diagram 9.248.2: Ponzo figure.
A geometrical-optical illusion, Mario Ponzo, Italy, (1882–1960).
Judging an object's size based on its background.
Perception of background influences perception of foreground.
The upper of the parallel lines is expanded with respect to the lower, showing the perspective significance of converging lines, e.g. railway rails.
9.25 Raindrops illusion
Raindrops illusion, stair rods
Raindrops are > 0.5 mm in diameter.
Supercooled rain, sleet, freezes on objects.
Thin clouds with weak updrafts cause drizzle drops, < 0.5 mm in diameter.
Raindrops from a sloping frontal surface are persistent in size, but not big.
The largest drops are formed in deep cloud layers with turrets containing strong updrafts.
These raindrops may contain ice crystals.
Very heavy rainfall may be seen as the optical illusion of "stair rods" that bounce after hitting the ground.
A falling raindrop seen from the side is indented in the middle from below, not tear-shaped as is often shown in illustrations.
This error is caused by the observation that just before a drop of water leaves a kitchen tap it hangs down in a teardrop shape.
However, when it becomes too heavy to remain attached to the tap, it falls and assumes a near-spherical shape, like a muffin with a concave lower surface.
Raindrop size is limited by the action of the slipstream when the raindrops achieve a high velocity of about 10 m / second.
However, larger drops may reach the ground in a downburst.
9.26 Refraction in water illusion
More than half fill a tall transparent glass with water.
Insert a pencil so that the side of the pencil touches the right hand top of the glass and the lower end touches the left inner wall of the glass, but not the bottom.
Looking down into the water, see the lower end of the pencil touching the wall.
At the same time move your left finger from up and down along the wall of the glass, until you think the finger points to the lower end of the pencil.
Look through the side of the glass to see the actual position of the pencil.
It is under your left finger.
The position of the left finger is the position of the image of the end of the pencil.
9.27 Retention of image illusion
See diagram 9.254.1: Retention of image (University of Melbourne).
Retention of image.
Two rings tilted with respect to each other are joined at a common point.
When the rings are placed upon the turntable and rotated it appears that the rings are in rotation with respect to each other.
9.28 Reversible figures illusion, Rubin vase illusion
Edgar Rubin, Denmark, (1886-1951). See diagram 9.247.1: Rubin vase.
A figure-ground illusion, the figures and ground compete to produce the profiles of two heads or a vase.
For most people the image fluctuates between the two although the image on the retina remains constant.
The perception by the brain keeps switching figure and background.
The primary decision the brain has to make is which shapes are objects and which are spaces between objects.
The brain wants closure, but the brain cannot make up its mind!
9.29 Rising coin illusion
Coin in a cup
See diagram 28.122.3: Rising coin illusion.
Pour water into a beaker until a coin at the bottom previously hidden by the side is visible.
9.30 Rising moon illusion
See diagram: Moon illusion.
See diagram 36.32.1: Rising Moon illusion.
See diagram 36.32.2: Ebbinghaus illusion.
The rising Moon appears to be much bigger on the horizon than the Moon high in the sky, but if you photograph both Moons they are the same size in the photograph.
The rising Moon optical illusion is caused by the adjacent buildings and trees that appear to be close to it.
This illusion may be called the angular size illusion and optical scientists are still discussing the best explanation for this common illusion.
It may be partly explained by the Ebbinghaus illusion where, for most people, black circle "A" looks bigger than "B".
The explanation that the moon illusion is caused by also seeing adjacent buildings and trees may be refuted by the observation of the moon illusion at sea.
9.31 Sense of touch
Make a sensitivity test stick by attaching one toothpick to a flat stick, e.g. craft stick, ice cream stick, popsicle stick, "Paddle Pop" stick.
Make five more sensitivity test sticks by attaching two toothpicks to a flat stick, 1 cm apart, 2 cm apart, 3 cm apart, 5 cm apart.
Press the toothpick points of the sensitivity sticks against the upper arm of a blindfolded student.
Ask the students to report on how many points are felt.
Test other parts of the body, e.g. fingertips, side of the face, legs, back, to find the most sensitive part of the body.
9.32 Size-weight illusion
Size-weight illusion, tactile illusion
The smaller object feels heavier, although both are the same scale weight.
Larger objects are generally heavier than small objects so the muscles are set in this expectation.
9.33 Slice of pie illusion
See diagram 9.246.7: Slice of pie.
A slice of pie is missing in the top diagram, but it reappears in the bottom diagram.
However, the bottom diagram could also show a pie with a slice cut out.
We never see a pie with a slice missing from this angle, unless we looked up through a glass table, so we are happy to accept the bottom diagram as the missing slice of pie.
9.34 Spinning picture illusion
Spinning picture, thaumatrope illusion, persistence of vision illusion
See diagram 28.143: Spinning picture.
In 1826, a two-sided disc, held at the ends with strings attached, had an image on each side, and was a popular toy in England.9.35 Spoon in water illusion
See diagram 9.127.2: Spoon in water illusion.
When the spoon leans against the side of the glass, the light from the spoon hits the water at an angle.
So it is refracted or bent, and the part of the spoon under water appears in a different location.
9.36 Sun and Moon diameter illusion
The Sun and the Moon appear to be greater in diameter at sunrise and sunset than when vertically overhead.
The appearance of the size of an object depends on the angle subtended by the object, α, and the distance from the object, d.
The distance may be known or may be estimated from the size of other familiar objects.
However, if there are no other objects in view the distance is usually underestimated.
We estimate that the sky is a saucer-shaped covering, so when the Sun or Moon is at its zenith, we estimate that it is closer than when at the horizon.
So its size is estimated as being greater that at the horizon.
9.37 Vertical- horizontal illusion
See diagram 9.246.8: Vertical-horizontal.
The brain overestimates the length of a vertical line relative to a horizontal line, but both are the same length.
9.38 Young girl-old woman illusion
See diagram 9.247.7: Young girl-old woman.
The chin of the young girl is the nose of the old woman.