School Science Lessons
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9.1.0 Optical Illusions
Table of contents

See: Optics (Commercial)

9.254 Optical illusions

9.246.1 Absent square

9.248.8 Afterimage

9.247.8 Barber pole

9.248.5 Bending pencil

28.122.2 Bent stick

9.246.9 Blivet

9.247.9 Clouds

28.5.4 Coin in cup

9.254.1 Count boxes

9.247.4 Duck-rabbit

9.248.1 Ebbinghaus

9.243.1 Feel two noses

9.246.4 Fraser spiral

9.246.3 Ghost coin

9.246.5 Herman-grid

9.247.3 Impossible square

9.247.5 Impossible triangle

28.109.1 Lateral inversion

36.20.6 Moon rising

36.20.4 "Man in the Moon"
28.4.6 Mirage

36.33 Moon, rising moon

Moving window

9.247.2 Müller-Lyer

9.246.2 Necker cube

9.248.3 Poggendorf

9.248.2 Ponzo figure

37.37.5 Raindrops

4.122 Refraction in water

9.254.1 Retention of image

9.247.1 Rubin vase

9.243 Sense of touch

9.248.6 Size-weight

9.246.7 Slice of pie

9.248.4 Spinning picture

28.122.2 Spoon in water

28.5.6 Stick in water

36.20.7 Sun & Moon

9.246.8 Vertical-horizontal

9.247.7 Young girl-old woman


9.254 Optical illusions
See: Optics (Commercial)
"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
3. Paradox illusions
How to make sense of neural signals.
See diagram 9.247.5: Impossible triangle, Penrose triangle, Penrose tri-bar
4. Fiction illusions
See diagram 9.247.9: Cloud shapes illusion
See diagram "Man in the Moon" illusion.

9.246.1 Absent square, Kanizsa triangle illusion
See diagram 9.246.1
: Absent square, square that is not there 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.

28.122.2 Bent stick illusion, See diagram 28.127.2: Spoon in water illusion
See diagram 28.122.2: Bent stick 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.

9.246.2 Necker cube
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?

9.246.3 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.

9.246.4 Fraser spiral, false spiral, twisted cord illusion
See diagram 9.246.4: Apparent spiral
Overlapping arcs appear to form a spiral, but the arcs are concentric circles.

9.246.5 Herman-grid illusion
See diagram 9.246.5: Dark and bright
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 whites than 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.

9.246.6 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.

9.246.7 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.

9.246.8 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.

9.246.9 Blivet illusion
See diagram 9.246.9: Blivet, devil's fork
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?

9.247.1 Reversible figures, face-vase, Rubin vase illusion
See diagram 9.247.1: Face-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!

9.247.2 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.

9.247.3 Impossible square illusion
See diagram 9.247.3: Impossible square
The surfaces and corners of the square are not in the same position as in a normal square.
Try to draw an impossible square!

9.247.4 Duck-rabbit illusion
See diagram 9.247.4: Duck-rabbit
The brain keeps switching from perception of a duck to perception of a rabbit.

9.247.5 Impossible triangle, Penrose triangle, Penrose tri-bar illusion
See diagram 9.247.5
: Impossible triangle
An "impossible triangle" can be constructed, so that by observing the construction 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.

9.247.7 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.

9.247.8 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.

9.247.9 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."

9.248.1 Ebbinghaus illusion
See diagram 9.248.1: Ebbinghaus
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.

9.248.2 Ponzo figure illusion
See diagram 9.248.2: Ponzo figure
The upper of the parallel lines is expanded with respect to the lower, showing the perspective significance of converging lines,
e.g. railway rails.
Perception of background influences perception of foreground.

9.248.3 Poggendorf illusion
See diagram 9.248.3: Poggendorf illusion
A straight line crossing a rectangle appears displaced.

9.248.4 Spinning picture, thaumatrope illusion
28.143 Spinning picture, persistence of vision illusion.

9.248.5 Bending pencil
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.

9.248.6 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.

9.248.7 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.

9.248.8 Afterimages
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."

9.254.1 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.