Ten of the Greatest Optical Illusions

8/22/2019 Ten of the Greatest Optical Illusions

1/9

Ten of the greatest optical

illusionsFrom normal-looking upside-down faces turninggrotesque to horizontal lines appearing to slope butare in fact parallel, RICHARD WISEMAN chooses histen favourite optical illusions

1. CHECKER SHADOW

Although it may seem impossible to believe, the squares marked 'A' and 'B' are actually exactly the same shadeof grey

This amazing illusion was created by Edward H Adelson from the Massachusetts Institute ofTechnology. Although it may seem impossible to believe, the squares marked 'A' and 'B' areactually exactly the same shade of grey! Your eyes and brain are constantly trying to figureout the colour of the objects around you, and in doing so automatically compensate for

shadows. The square marked 'B' is in the shadow cast by the green cylinder, while thesquare marked 'A' is outside of the shadow. Your eyes and brain see that the two squares


2/9

are the same shade of grey, but then think, 'Hold on - if a square in a shadow reflects thesame amount of light as a square outside of the shadow, then in reality it must be a muchlighter shade of grey.' As a result, your brain alters your perception of the image so that yousee what it thinks is out there in the real world.

2. GRID

As you move your eyes around the image, dark dots quickly appear and disappear at the intersections. However,whenever you look directly on any intersection, the dark dots vanish

The original version of this illusion was first reported by German physiologist LudimarHermann in 1870, and simply involves a white grid on a black background. As you moveyour eyes around the image, dark dots quickly appear and disappear at the intersections.However, whenever you look directly on any intersection, the dark dots vanish. For years itwas widely believed that the illusion worked because of 'lateral inhibition' - the term used todescribe the complex way in which the cells on the back of the retina respond to areas ofblack and white. There is, however, little point in explaining the theory. Why? Because a fewyears ago it was shown to be completely untrue, and thus the explanation for the illusionremains a mystery...

3. THATCHER


3/9

This upside-down picture of ex-Prime Minister Margaret Thatcher looks perfectly normal (left) but when you rotatethe photograph the right way up, the face will appear grotesque (right)

This upside-down photograph of ex-Prime Minister Margaret Thatcher looks perfectlynormal. However, when you rotate the photograph the right way up the face will appeargrotesque, thus proving that 'the lady's not for turning'. The grotesque face is due to the eyesand mouth being inverted. But why don't you spot this when the photograph is upsidedown?There are specific parts of your brain dedicated to face perception. However, because yourarely encounter upside-down faces, these parts of your brain work best with upright faces.When presented with an upside-down face, your brain is able to identify the different parts ofthe face, such as the eyes and mouth, but unable to perceive the relationship between theseparts; hence it doesn't spot the distorted face.


4/9

4. TABLETOPS

In this classic illusion, the two tabletops look completely different shapes but are actually identical

In this classic illusion, created by Stanford University psychologist Roger Shepard, the twotabletops look completely different shapes but are actually identical. The illusion works fortwo reasons. First, vertical lines tend to look relatively long while horizontal lines tend to lookrelatively short. Also, the legs induce a sense of perspective, causing the back of thetabletop on the left to appear to be much further away than the back of the tabletop on the

right. Your brain alters your perception of the tabletops, making the left one appearcompletely different to the right one.

5. CAFE WALL

The horizontal lines in this image appear to be sloping, but in reality they're parallel to one another


5/9

The horizontal lines in this image appear to be sloping, but in reality they're parallel to oneanother. Why does it work? Although it's easy to see the mortar line between two black tilesor two white tiles, it's much harder to see the mortar line between a white tile and a blackone. Your brain fills in the gap by seeing it as part of either a white or black tile. This, in turn,makes the tiles look wider at one end than at the other, creating the illusion of a series ofwedge-shaped tiles, which makes the lines appear to slant.

6. LEANING TOWERS

These two photographs of the Leaning Tower of Pisa are identical, yet when your eyes and brain see two towersthat are parallel, they assume that they must be diverging as they rise into the air

These two photographs of the Leaning Tower of Pisa are identical, yet the tower on the rightappears to lean more than the tower on the left. Created by Frederick Kingdom andcolleagues from McGill University in Montreal, the illusion works because your eyes andbrain treat the two photographs as if they're part of a single scene. If the two towers reallywere next to one another and rising at the same angle, they'd appear to converge due toperspective. So when your eyes and brain see two towers that are parallel, they assume thatthey must be diverging as they rise into the air, and thus create the resulting illusion. Each

year the Neural Correlate Society holds the Best Illusion of the Year Contest to find new andwonderful illusions. The Leaning Tower illusion won the contest in 2007.


6/9

This image looks like physicist Albert Einstein. However, move a few feet away from the screen and suddenly it'lltransform into Marilyn Monroe

7. EINSTEINThis amazing image looks like physicist Albert Einstein. However, move a few feet awayfrom the screen and suddenly it'll transform into Marilyn Monroe. The work of Aude Olivaand her colleagues at the Massachusetts Institute of Technology, the illusion was created inthree steps.First, the researchers obtained a photograph of Marilyn Monroe and removed the fine-grained facial features, such as any wrinkles or other blemishes.Second, they obtained a photograph of Albert Einstein and removed the more coarsefeatures, such as the shape of the mouth or nose.Finally, the two images were superimposed on top of one another. Because the fine-grainedfeatures are visible close up, the image looks like Albert Einstein when you're just a fewinches away from the page. However, move a few feet away and suddenly only the coarsefeatures are visible, magically transforming the image into Marilyn Monroe.


7/9

8. DUCK-RABBIT

This image is known as a 'bistable' image. You won't be able to see both pictures at the same time and willinstead flip between seeing the duck and then the rabbit

When you first look at this picture you'll probably see a rabbit facing to the right. However, ifyou continue to look at the picture it'll flip to become a duck looking to the left. This is known

as a 'bistable' image. You won't be able to see both pictures at the same time and willinstead flip between seeing the duck and then the rabbit. This illusion was originallypopularised in 1899 by American psychologist Joseph Jastrow, who used it to make thepoint that we 'see' with our brains as well as our eyes.


8/9

9. ROTATING SNAKES

The effects of this image works best in peripheral vision, so when you stare at one of the coils it will appearstationary while those around it will appear to rotate

Although the coils in the image appear to be rotating, in reality they're completely stationary.The effect works best in peripheral vision, so when you stare at one of the coils it will appear

stationary while those around it will appear to rotate. This wonderful illusion was created byJapanese psychologist Akiyoshi Kitaoka from Ritsumeikan University in Kyoto. Visionexperts aren't exactly certain why it works; however, their research has revealed that theshading of the segments that make up the rings is crucial. These segments are arranged ina repetitive pattern consisting of a relatively dark area (yellow) followed by a brighter one(white), then a less bright one (blue), and finally the darkest area (black). Information fromhigh-contrast parts of the image (yellow-white, white-blue and blue-black) travels to the brainfaster than that from low-contrast parts (blue-black). It's believed that this 'staggered'information mimics the type of input that the eyes and brain receive when they see genuinemotion, and so you end up believing that you're looking at actual movement.


9/9

10. IMPOSSIBLE TRIANGLE

The dice appear to form an impossible triangle. If the position of the camera was to shift slightly, you would seethat in reality the 'triangle' is actually created by three lines of dice arranged in the shape of the letter 'Z'

The dice appear to form an impossible triangle. Your eyes and brain are fooled because theyassume that all of the corners of the triangle are touching one another. In reality, thephotograph has been taken from a very specific angle. If the position of the camera was toshift slightly, you would see that in reality the 'triangle' is actually created by three lines ofdice arranged in the shape of the letter 'Z'. As such, one of the 'corners' in the photographactually consists of two ends that are a very long way apart. This idea is based on a drawingof an impossible triangle originally created by physicist Roger Penrose in 1954.

Documents

Ten of the Greatest Optical Illusions