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Case-lesson "Optical Illusions"
Case-lesson "Optical Illusions"

Category: Science, nature and human

The level (grade): 8 +

Subject: Natural phenomena

Objective: To learn about the relationships of physical, chemical, biological and psychological phenomena on the example of optical illusions

What information is waiting for me here?

    • Why are optical illusions such unusual?
    • Why do we see mirages?
    • What is Fata-morgana?
    • What fundamental and practical knowledge will be useful to me?
    • How do animals use optical illusions?

Is it possible to find a practical application of optical illusions in the modern world?

6 scans of the subjects, phenomena and practices:

Optical illusions surround people everywhere. Therefore, you should be able to navigate in this sea of illusions. And critical thinking has to become the basis of this ability, because you see something, but in fact, there is something entirely different.

But if you apply a systematic approach and methods of effective information analysis, they will help understand causes, characteristics and consequences of optical illusions.

But if we turn creativity, we can successfully use illusions in fashion or art. Perhaps due to the creativity and non-standard, non-linear thinking you can use them in other life areas! At these competencies this case-lesson is aimed.

Look at these lines. Do you think they are parallel?


At first glance it seems they are not. But the lines are parallel! This optical illusion divides our thinking.

Optical illusion is a bug in visual perception. It is caused by the inaccuracy of unconscious processes of visual image correction (moon illusion, incorrect lengths assessment, banner blindness etc.) and natural causes (mirage, "broken spoon" in a glass of water, St. Elmo's fire). Every day we face many of them. Let's try to understand this "illusory world" and find an explanation for at least some of them.


So, an optical illusion is a bug in visual image perception. Why does it occur? In terms of psychology and physiology the mechanism is the following: an observer consciously or unconsciously gives an inadequate explanation of reality picture he observes. He sees not what it is really in there. The information gathered by the eye is processed in the brain to give a percept that does not tally with a physical measurement of the stimulus source.
This explains some of the "sensations" in the media about seeing human head on Mars, etc.
At the same time accidentally created spots are sometimes used by psychologists to determine the properties of examined intelligence, which is forced to experience the optical illusion (Rorschach test).

Psychology explains, first of all, artificial illusions. Let’s look at their main types.

Illusion of color perception

Although we "see" with our brain and distinguish colors with its help, eyes have very important and irreplaceable function.
They percept seven colors: red, orange, yellow, green, blue, indigo and violet. Some receptors of the retina are agitated by twilight light, others – by bright only. Color vision and color perception are associated with them.

How do eyes distinguish colors? Here's how it is explained by the theory of color vision: eye contains three types of nerve cells that respond respectively to red, green and blue colors. Thus, if all three types of nerve cells receive identical stimuli, we see white color. If mostly green light gets into the eye, the cells responsible for the green part of the spectrum are more irritated than others, and we see green color. When the object is yellow “green” and “red” cells are stimulated. This is also due to the fact that every color in the spectrum has its own wavelength (in nm):


One of the color perception illusions was described in the 1995 by MIT professor Edward Adelson ("Checker shadow illusion"). He paid attention to the fact that color perception essentially depends on the background and the same colors on different backgrounds are perceived as different, even if they are close and we see them both simultaneously:


Moron Boer Rosa Illusion: Each rectangle’s right side (in the triangle) appears darker than the left, although they have the same brightness:


Contrast illusion

Ebbinghaus illusion: Observe the two sets of circles below. Which of the red circles is larger?


You may be surprised to find that they are exactly the same size. The deception occurs because of the size of the surrounding blue circles and their relative distance from the central red circle. These adjustments cause the brain's visual perception system to distort the relative size of the inner circles. Another factor is the "completeness" of the surrounding circles; if we removed a few of the blue dots or spaced them out, the illusion would not persist.

Mach bands. A Mach bands is an optical illusion named after the physicist Ernst Mach. It exaggerates the contrast between edges of the slightly differing shades of gray, as soon as they contact one another, by triggering edge-detection in the human visual system.


Distorted illusions

The lines are parallel, although the brain perceives the image differently, twisting it:


The diagonal checkered squares at the larger grid points make the grid appear distorted.


Depth perception

Depth perception is the visual ability to perceive the distance of an object. Depth sensation is the corresponding term for animals, since although it is known that animals can sense the distance of an object (because of their ability to move accurately, or to respond consistently, according to that distance), it is not known whether they "perceive" it in the same subjective way that humans do.

A striking example - illusion of shimmering lattice:


The illusion of perception size

Illusions often lead to completely wrong quantitative estimates of the real geometrical quantities. It turns out that one can wrong on 25% or more about the size of the object, if not to check it with a line. Geometric evaluation of real values strongly depends on the nature of the background image. This applies to lengths, areas, and radii of curvature, angles, shapes and more.
Ames room is one of the examples of this illusion.
The Ames room is a distorted room that is used to create an optical illusion. Likely influenced by the writings of Hermann Helmholtz, it was invented by American ophthalmologist Adelbert Ames, Jr. in 1934, and constructed the following year.



The Ames room is viewed with one eye through a pinhole such as to avoid any clues from stereopsis, and it is constructed so that from the front it appears to be an ordinary cubic-shaped room, with a back wall and two side walls parallel to each other and perpendicular to the horizontally level floor and ceiling. However, this is a trick of perspective and the true shape of the room is trapezoidal: the walls are slanted and the ceiling and floor are at an incline, and the right corner is much closer to the front-positioned observer than the left corner (or vice versa).

Reversible figures illusions

Reversible figure is a kind of optical illusion in which eye position affects the nature of the perceived object:


Color blindness may be considered another optical illusion. Strictly speaking, it is a disease, and the inability to distinguish certain colors is its manifestation. But this incapability complicates driving and many other tasks where color symbolism is important. It is becoming more and more difficult to solve them.

Color blindness can be a hereditary disease, which can be explained in terms of genetics, and it can also be acquired. The cause of acquired color blindness is destruction of the retina. This blindness develops only in the eye area where the retina is affected.

There may be a few variants of color blindness; full color blindness is very rare. In the case of absence of one of the visual pigments in the retina, a person is able to discern only two primary colors. Such disease is called dichromacy.

Protanopia is a severe type of color vision deficiency caused by the complete absence of red retinal photoreceptors.

Deuteranopia is a type of color vision deficiency where the green photoreceptors are absent. It affects hue discrimination in the same way as protanopia, but without the dimming effect.

Tritanopia is a very rare color vision disturbance in which there are only two cone pigments present and a total absence of blue retinal receptors. Blues appear greenish, yellows and oranges appear pinkish, and purple colors appear deep red.

People who suffer from these types of disorders perceive colors as the following:



Moving illusions

One and the same animation can play rotary motion clockwise, counterclockwise or interchangeably (perform oscillatory motion). A prime example is this ballerina: in which direction is she spinning?


The spinning girl is a form of the more general spinning silhouette illusion. The image is not objectively “rotating” in one direction or another. It is the two-dimensional image that is simply shifting back and forth. But our brains aren’t capable of interpreting two-dimensional representations of the world, only the actual three-dimensional world.

The easiest way to understand it is to mark the left leg and arm of the ballerina with red lines, and the right hand and leg – with blue lines:


The master of this optical illusion was Maurits Cornelis Escher, the Dutch graphic artist. One of his famous illusions is a lithograph "Waterfall":


This lithograph has inspired the modern artists to create the "incredible waterfall":

How does this waterfall "work"?


Try to create an optical illusion using the described techniques.

Protective coloring is one of the natural means of protection and optical illusion. How much time will you need to find a lizard on the left photo, and an owl - on the right?


Even gradual color change is a natural protective coloring and optical illusion. Protective coloration is the reaction of a living organism adaptation mechanism for natural selection and a way to increase the chances of survival. But every coloring has its time and place: seasonal color change also belongs to this area. The white hare is not so noticeable in the snow, right?


The top of protective coloration is mimicry. In the process of evolution animals have "learned" to imitate other types of beings and even inanimate objects.

Color mimicry

The most common and well-known phenomenon is general harmony of animals’ coloring with places of their living. White body color is spread among arctic animals. Some have the same coloring during the whole year: polar bear, snowy owl. Others who live in areas where there is no snow in summer usually have their brown coloring changed to white only in winter: foxes, hares and ermines.


Another example of the most common security or harmonious coloring is observed in deserts of the world. Living creatures there represent a huge number of sand color shapes and its various shades; this occurs not only on small creatures, but even on such large animals as lions. During the collective mimicry the large group of small-sized organisms slips in the complete cluster.
The same phenomenon in the widest ranges is represented by the fauna of the sea: organisms that live on the seabed are extremely difficult to notice because of their colors and contours of body surfaces.

Form mimicry

There are cases when animals become extraordinary similar with the objects of their environment not only in color but also in shape. This is called inheritance. Especially there are many such examples among insects.

Caterpillars of butterflies live on the branches of plants, with which they are similar in color, and have a habit of attaching rear legs to them, stretch and keep their bodies still in the air. It is very hard to discern them:


A fantastic variant of form mimicry are shown by tropical stick insects: some inherit color and body shape from dry sticks, a few centimeters in length, others – from leaves.

Collective mimicry

During the collective mimicry a large group of small-sized organisms slips in a complete cluster to create an image of a large animal (sometimes a certain species) or plant:



Search the Internet for information and prepare either a short presentation on the biological optical illusions or a task like "find some living being that “have hidden” in the picture


Pigments are specific substances that are responsible for colors of eyes, hair, skin, leaves, petals, wings of butterflies, birds’ feathers. The most “relative” to human pigment is melanin. It is responsible for hair and skin color, tan and pallor:



And what natural pigments are responsible for optical illusions? And which animal may carry the title of "illusionist record-breaker"? 

In the world of optical illusions chameleon is a unique phenomenon. Its color change is due not only to defensive reaction. In order to survive there are a range of reasons, including psychological, biological and chemical characteristics:

Oddly enough, the color does not always depend on the environment: chameleon just doesn’t pay it any attention. The skin of the animal is transparent. There are black, red and yellow cells – chromatophores under it. They contain coloring substances – pigments. When they are compressed or extended, it creates the impression that the chameleon changes color.

What makes these cells work? When the animal is angry or frightened, its nervous system sends signals to these cells. Rage causes a dark color, excitement or fear – pale shades and yellow spots.

Sunlight also affects the chameleon’s coloring. The sun makes the cells dark or even black. High temperature without the direct sun rays causes green color, and in the darkness the chameleon becomes creamy with yellow spots:


Factors such as emotion, temperature, light are the ones forcing animals’ nervous system to change the condition of colored cells, not the environment. These color changes help chameleons hide from their enemies such as snakes and birds. And since the animal moves extremely slowly, this kind of protection is crucial for its survival.

What are chromatophores? They are reflective cells that contain pigments. They are present in amphibians, fish, reptiles, crustaceans and cephalopods. Chromatophores are contained in tissues of plants and tincture them. The best known pigment is chlorophyll, contained in chloroplasts of plants:


How does the color change? The melanophores play a crucial role in colour change.

They are large, star-like cells with long “arms” (dendrites) that extend towards the skin’s surface.

Colour change occurs due to the movement of “packets” of melanin pigment (melanosomes) within the melanophores.

When melanin pigment is aggregated within the centre of the cell, the skin appears very pale, whereas when it is dispersed through the arms of the melanophores towards the skin’s surface, the animal appears dark. Because the arms of the melanophores extend between and over the other types of chromatophore (generating yellows, reds, blues, etc.), varying the degree of dispersion of the melanin can conceal or reveal those chromatophores, thereby varying the animal’s colour.

Colour change may also occur due to changes in the spacing of the stacks of platelets or crystals within the iridophores, which changes the way they reflect and scatter light, and therefore their colour.


Many optical illusions, natural and synthetic, have a logical explanation in terms of physics. Well, let’s proceed to "exposure session"!


A mirage is a naturally occurring optical phenomenon in which light rays are bent to produce a displaced image of distant objects or the sky. In contrast to a hallucination, a mirage is a real optical phenomenon that can be captured on camera, since light rays are actually refracted to form the false image at the observer's location. What the image appears to represent, however, is determined by the interpretive faculties of the human mind. For example, inferior images on land are very easily mistaken for the reflections from a small body of water.

Mirages can be categorized as "inferior" (meaning lower), "superior" (meaning higher) and "lateral mirages", one kind of superior mirage consisting of a series of unusually elaborate, vertically stacked images, which form one rapidly changing mirage:


Fata Morgana

It is a very complex and rare superior mirage, which consists of several forms of mirages. Remote objects are seen many times and with various distortions:


It appears with alternations of compressed and stretched zones, erect images, and inverted images. A Fata Morgana is also a fast-changing mirage.

As a result of the reflection and refraction of sunlight, actually existing objects on the horizon give several distorted images. Such images partially overlap and quickly change over time, which creates a bizarre picture of Fata Morgana.

St. Elmo's fire

It can also be attributed to optical illusions. But the nature of this phenomenon is electric. St. Elmo's fire is named after St. Erasmus of Formia, the Catholic patron of sailors. The phenomenon sometimes appeared on ships at sea during thunderstorms and was regarded by sailors with religious awe for its glowing ball of light – hence the name. Sailors may have considered St. Elmo's fire as a good omen (as in, a sign of the presence of their patron).

The phenomenon can occur on a plane that got into the cloud of volcanic ash. It is a kind of corona discharge:


Aurora (polar light)

It is incredibly beautiful and impressive atmospheric phenomenon that has a number of "physical" explanations:

Why does aurora occur?

Auroras are produced when the magnetosphere is sufficiently disturbed by the solar wind that the trajectories of charged particles in both solar wind and magnetospheric plasma, mainly in the form of electrons and protons, precipitate them into the upper atmosphere (thermosphere/exosphere), where their energy is lost. The resulting ionization and excitation of atmospheric constituents emits light of varying colour and complexity. The form of the aurora, occurring within bands around both polar regions, is also dependent on the amount of acceleration imparted to the precipitating particles. Precipitating protons generally produce optical emissions as incident hydrogen atoms after gaining electrons from the atmosphere. Proton auroras are usually observed at lower latitudes.

Red: At the highest altitudes, excited atomic oxygen emits at 630.0 nm (red); low concentration of atoms and lower sensitivity of eyes at this wavelength make this color visible only under more intense solar activity.

Blue: At yet lower altitudes, atomic oxygen is uncommon, and molecular nitrogen and ionized molecular nitrogen takes over in producing visible light emission; radiating at a large number of wavelengths in both red and blue parts of the spectrum, with 428 nm (blue) being dominant..

Ultraviolet: Ultraviolet light from auroras (within the optical window but not visible to virtually all humans) has been observed with the requisite equipment.

Yellow and pink are a mix of red and green or blue. Other shades of red as well as orange may be seen on rare occasions; yellow-green is moderately common.

Green is the primary colour:


Not in all parts of the world can we see this unique phenomenon, but only near the magnetic poles. This is because the geomagnetic field of the Earth turns away the huge amount of the flow of charged particles coming from the Sun, and only some of them enter the Earth's atmosphere and only over the poles. Aurora can be observed by the residents and visitors of Norway, Sweden (Kiruna), Finland (northern part), Iceland, Canada (Great Lakes in Ontario), Russia (Kola Peninsula).

Is there aurora on other planets? Yes, there is! This phenomenon can be observed on Venus, Saturn, Jupiter and Mars. The explanation is almost the same as for the Earth. Examples:


But optical phenomena and illusions, which can be explained by the laws of physics, have not only the space or planetary scale. They exist at the household level. Distorting mirror is a variant of the optical illusion with "physical" explanation. Concave or convex mirror surface reflects the rays differently, which leads to a distorted picture:


You can try to create your own "laughter room online" using special programs to distort photos. There are plenty of such programs; one of them is:


Why do objects in water appear larger or smaller? Why a spoon in the glass appears distorted? The reason is the refraction of light.  Refraction can be seen when looking into a bowl of water. Air has a refractive index of about 1.0003, and water has a refractive index of about 1.3330. If a person looks at a straight object, such as a pencil or straw, which is placed at a slant, partially in the water, the object appears to bend at the water's surface. This is due to the bending of light rays as they move from the water to the air. Once the rays reach the eye, the eye traces them back as straight lines (lines of sight). The lines of sight (shown as dashed lines) intersect at a higher position than where the actual rays originated. This causes the pencil to appear higher and the water to appear shallower than it really is. The depth that the water appears to be when viewed from above is known as the apparent depth. Conversely, an object above the water has a higher apparent height when viewed from below the water. This phenomenon is

described by Snell’s law. It describes the refraction on the verge of transparent environment:



What optical illusions are there in your everyday life, and which can be explained by the laws of physics? Prepare a little message on the subject.
Fashion / People

Optical illusion in fashion is not a new idea. Corset with a fluffy skirt make the figure slimmer; inflated waistline visually makes legs longer; huge ruff collar creates the illusion of a little head…

Illusions in fashion can cause considerable debate. For example, the global debate “what is the color of this dress”. This dress was jokingly christened the "Schrödinger dress":

In efforts to make figure visually perfect without changing the body, optical illusions are indispensable. Modern designers have learned to skillfully use them. What optical illusions are used in fashion?

Vertical–horizontal illusion

The vertical–horizontal illusion is the tendency for observers to overestimate the length of a vertical line relative to a horizontal line of the same length.This involves a bisecting component that causes the bisecting line to appear longer than the line that is bisected.

This illusion is most commonly used to determine the proportions of upper and lower parts of clothing. Equivalent lengths of a skirt and blouse are not perceived as equal by eye. The length skirts are visually slightly shifted upwards, and this small difference causes confusion because eyes begin to compare: what is longer and what is shorter. The difference must be more explicit to understand what is in need of lengthening: a blouse or a skirt:


Filled-space illusion

Sometimes it happens that space on the outfit, full of details and décor, seems bigger than equal to it but blank. Hence, it is better to avoid the accumulation of details in places where the figure is undesirable to look expanded. Or, conversely, accent right places with decor:


Illusion of acute angles

The distance between the sides of acute angle seems longer than it actually is, and the distance between the sides of obtuse angle is underestimated.

This mainly concerns the neck design. The wide triangular neckline makes broad shoulders narrower, and long and narrow, conversely, expands them:


The illusion of contrast

This illusion is used exceedingly widely in costume design. For example, in a large hat head seems smaller than in a small one; thin neck, surrounded by a wide neckline, seems even thinner; thin hand in a wide sleeve seems narrower, and heavily laced waist will make wide hips look even wider:


The illusion of volume reduction achieved by dividing a costume with vertical contrasting color fabrics 

This illusion is based on asymmetry, for example, can be created in the following way: the left half of the outfit is white, right is black; left sleeve is black, right is white. Through vertical division it is possible to reduce the volume of a broad figure, adding a certain elegance and dynamism to it:


The illusion of space with a gradual reduction, compression, reducing of fabric pattern

This effect is the most common in models of 'op art' style. For example, a large cell gradually becomes smaller and as if “sent into space”. And the part with larger cells seems bigger and more volumetric than the part with smaller cells.

So if you do not want to draw attention to some parts of your figure, don’t place large patterns on it. Just place small patterns, and a large one, growing, will gently transfer the attention on other part of your figure:


The master of illusion was the most famous designer Elsa Schiaparelli. She was an Italian fashion designer, founder of prêt-à-porter style. Along with Coco Chanel, her greatest rival, she is regarded as one of the most prominent figures in fashion between the two World Wars. Schiaparelli's designs were heavily influenced by surrealists like her collaborators Salvador Dalí and Jean Cocteau. Her clientage included the French-American heiress Daisy Fellowes and actress Mae West.

Schiaparelli was one of the first designers to develop the wrap dress, taking inspiration from aprons to produce a design that would accommodate and flatter all female body types. Her design, which first appeared in 1930, offered a two-sided model with armholes on each side, brought together in the front of the garment and wrapped and tied at the waistline.


Her works are amazing, she is still an icon!


The most famous in the fashion area optical illusion is the illusion of strips. Try to experiment with it (using pictures, slides, photos, cameras (webcams)). Compose a mini-report with recommendations based on the results of your experiment.


The play of light and shade gives volume to a flat image. The sun's rays, which fall at a certain angle, “vitalize” and "animate" a picture. Art is simply not possible without optical illusions. This is the art of optical illusion.


Mural art is one form of out-door painting, but, unlike street art, paintings in this genre seem to be more ambitious. This is one of the modern arts, along with street art that is becoming increasingly popular:


Street Art

This type of art gathers graffiti, stencils, installation and much more. This is where the urban environment is changing beyond recognition! This complex urban landscape is the perfect canvas for it:


Anamorphic illusion

This art form is 3d-painting when the image acquires volume when it is viewed at the certain angle:

"Animation on paper"

Our brain "sticks together" the successive frames and during the fast review turns movements into illusion:


It is one of the most “technological” arts where a set of technology for accurate recording, playback and reformatting the wave fields of optical electromagnetic radiation are used. This is the specific photographic procedure, during which three-dimensional objects’ images, which are extremely similar to the real ones, are made using a laser.

For example, this is the same hologram, taken from different angles:


Is it possible to make a hologram by yourself? Yes, it is! There are several techniques and methods. For example, you can use your smartphone:

Laser show

This is the most modern, the most knowledge-intensive and high-tech illusion art appearance. The canvas is either the night sky or historical buildings. And artists are a team of professionals working on a filigree image. Movement, color, volume, plasticity – they all are the result of using a laser beam and light effects:


You can try to make a paper or hologram animation yourself. Complexity will depend on your skills and patience. Good luck!

Lesson summary:




What competencies were obtained:

Science adeptness

Information and computer technology

Skills of efficient information processing

Inventive skills


What three sites helped you find information?





To help student and coach:







To help student and coach:




















http: https://ru.wikipedia.org/wiki/%D0%9C%D0%B5%D0%BB%D0%B0%D0%BD%D0%B8%D0%BD%D1%8B//tribayana.ru/?p=1856


Locations of the lesson:


Case lesson takes place in the classroom. It is possible to conduct classes at the museum, park, library...



Teams of boys and girls.

The score of the competition between teams was

Tasks for them:    

1.Using knowledge, try to use one of the optical illusions to develop interesting interior details

(Room, class and etc.). Suggest a "concept."

What methods for creating optical illusions did you use?

2.Try to solve seemingly unpretentious but tricky geometric problem:


Why does empty space suddenly appear after rearrangement of parts?


 Answer: really, the figure is not a triangle, and "hypotenuse" is a broken line.

But you have been waiting to see a triangle and you’ve seen it.




Conduct a mini-survey. Carefully examine your surroundings: class, house, familiar landscape. Try to "see" optical illusions that are hidden in all of it. What are the types of illusions, on what are they based? Make a small report or presentation of the study.


Duration of the Case:

90 minutes (double lesson).


Ability to circuit classes with student-double for:



The acquired results and competencies:

• The ability to organize and analyze complex

multifaceted information.

• The ability to find different scientific justification

effects and their practical application.

• Obtaining specific knowledge in physics, chemistry, biology

and other sections (shown in the scans).



Protective coloration, mimicry, Mural, Street art, op art, illusion, mirage, Fata Morgana, St. Elmo's Fire, aurora, Snell’s law, chromatophores.



Hrabovska Larysa



Took part in case upgrade:


The End

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