7  Optical Illusions: Puzzles that rely on the manipulation of visual perception.

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7.1 Ambiguous Figures

📖 Images that can be interpreted in multiple ways.

7.1.1 Problem

A black and white optical illusion image of a vase or two faces staring at each other in profile.

  • Hint:
    • Gestalt psychology suggests that the human eye and brain tend to organise visual information into groups and patterns, often based on proximity and similarity.
  • Answer:
    • The image can be seen as either a vase or two faces depending on how you interpret the black and white shapes.

7.1.2 Problem

An optical illusion of a checkerboard with either a gray square in the center or a white square in the center with four gray squares around it.

  • Hint:
    • Our brain tends to fill in missing information and create a coherent percept.
  • Answer:
    • The center square is the same shade of gray, but is perceived differently depending on the surrounding squares.

7.1.3 Problem

An optical illusion of a woman’s face that also resembles a horse.

  • Hint:
    • Our brains can find multiple interpretations of an image and switch between them.
  • Answer:
    • The image can be seen as either a woman’s face or a horse depending on which part of the image you focus on.

7.1.4 Problem

An optical illusion where a set of stairs appears to be going both up and down at the same time.

  • Hint:
    • Our visual perception can be influenced by the context in which an object is presented.
  • Answer:
    • The steps can be seen as either going up or down depending on which way you imagine yourself walking on them.

7.1.5 Problem

An optical illusion of a moving spiral that appears to be rotating in opposite directions.

  • Hint:
    • Our brains can be tricked by certain patterns and movements.
  • Answer:
    • The spiral actually remains stationary, but the alternating black and white sections create the illusion of movement.

7.2 Impossible Objects

📖 Objects that cannot exist in the real world due to logical or physical contradictions.

7.2.1 Problem

An artist creates a self-portrait but the image is not of themselves. How is this possible?

  • Hint:
    • Consider the medium used to make the image.
  • Answer:
    • The artist creates a sculpture of themself.

7.2.2 Problem

A cube has six square sides. If you saw off one corner, how many sides does it have?

  • Hint:
    • Count the number of surfaces.
  • Answer:
    • Still six.

7.2.3 Problem

A farmer has 12 sheep and all but 4 die. How many sheep does he have left?

  • Hint:
    • Think about the meaning of the word “left.”
  • Answer:
    • Four.

7.2.4 Problem

If you remove the first letter of every word in this sentence, will you be able to read it?

  • Hint:
    • Focus on the structure of the sentence.
  • Answer:
    • Yes.

7.2.5 Problem

A man is found dead in the desert, next to him is a bag full of gold and a canteen of water. There are no footprints around him, and no signs of a struggle. How did he die?

  • Hint:
    • Consider the location and the items found with the body.
  • Answer:
    • He choked on a gold coin.

7.3 Perspective Illusions

📖 Images that create the illusion of depth or three-dimensionality on a flat surface.

7.3.1 Problem

Which triangle is bigger?

  • Hint:
    • The two triangles are identical.
  • Answer:
    • They are the same size.

7.3.2 Problem

How many black dots are there?

  • Hint:
    • Count the white dots.
  • Answer:
    • 12 black dots.

7.3.3 Problem

Which of the following circles has the larger area?

  • Hint:
    • Consider the grids behind the circle.
  • Answer:
    • The circle on the right.

7.3.4 Problem

Which vertical line is longer?

  • Hint:
    • Use a ruler to measure the lines.
  • Answer:
    • The lines are the same length.

7.3.5 Problem

Which of the lines is longer?

  • Hint:
    • Consider the angles of the lines.
  • Answer:
    • The top line is longer.

7.4 Motion Illusions

📖 Images that appear to move or change when viewed, even though they are static.

7.4.1 Problem

A series of black dots appear on a white background. Some people see the dots moving in an irregular pattern, while others see them remaining stationary. What is the reason for this difference in perception?

  • Hint:
    • The dots themselves are not actually moving.
  • Answer:
    • The pattern of dots creates an optical illusion known as the Hermann grid illusion. The brain interprets the intersections of the white lines as gray dots that appear to move or flicker.

7.4.2 Problem

An image of a rotating ballerina appears to change direction when viewed from different angles. How is this effect achieved?

  • Hint:
    • The image is not actually rotating.
  • Answer:
    • The ballerina image is a series of static frames that create the illusion of movement when viewed in sequence. This effect is known as the motion illusion or stroboscopic effect.

7.4.3 Problem

A spiral pattern is drawn on a flat surface. When a person stares at the center of the spiral for a few seconds and then looks away, they see an afterimage of the spiral rotating in the opposite direction. What causes this illusion?

  • Hint:
    • The brain’s response to prolonged visual stimulation.
  • Answer:
    • Staring at the spiral causes the neurons in the retina to adapt to the pattern. When the person looks away, the neurons gradually return to their normal state, creating an afterimage that appears to move in the opposite direction.

7.4.4 Problem

A series of concentric circles is drawn on a piece of paper. When the paper is rotated quickly, the circles appear to undulate or ripple. What is the explanation for this illusion?

  • Hint:
    • The brain’s processing of visual information.
  • Answer:
    • The illusion is known as the Pinna-Brelstaff illusion. As the paper rotates, the brain receives conflicting visual information from the moving circles. This causes the brain to interpret the circles as undulating or rippling.

7.4.5 Problem

A person wearing glasses with a certain type of lens sees objects moving in a strange and distorted way. What is causing this distortion?

  • Hint:
    • The shape of the lens.
  • Answer:
    • The lenses in the glasses are cylindrical, which causes objects to appear elongated or compressed in one direction. This distortion is known as astigmatism and is caused by an uneven curvature of the cornea or lens of the eye.

7.5 Color Illusions

📖 Images that trick the eye into perceiving colors differently than they actually are.

7.5.1 Problem

In the image below, do the two squares marked A and B appear to be the same shade of gray?

  • Hint:
    • Consider the surrounding colors.
  • Answer:
    • No. Square B is slightly darker than Square A due to the effect of the surrounding colors.

7.5.2 Problem

In the image below, how many colors do you see?

  • Hint:
    • Focus on the center of the image.
  • Answer:
    • It is actually a black and white image. The eye perceives colors due to the contrast between the black and white stripes.

7.5.3 Problem

What color is the dress in the famous viral image?

  • Hint:
    • It’s a matter of perception.
  • Answer:
    • There is no definitive answer. Some people see it as white and gold, while others see it as blue and black.

7.5.4 Problem

In the image below, do the vertical lines appear parallel to each other?

  • Hint:
    • Use a ruler or straight edge to check.
  • Answer:
    • No. The lines converge at infinity, creating the illusion of parallelism.

7.5.5 Problem

In the image below, which square is larger?

  • Hint:
    • Pay attention to the perspective.
  • Answer:
    • The square on the left is larger. The image creates an illusion of depth, making the square on the right appear smaller.