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Optical Illusions

Crystal Risko
Product Developer

When students study eye structure, how vision works, or how different people perceive images, optical illusions can be an engaging subject to capture their attention. Also known as visual illusions, optical illusions are images perceived differently than they actually are. While we refer to these as optical illusions, in most cases the trick occurs in the brain and its reception or perception of the information received.

What creates the illusion?

Some optical illusions occur before light enters the eye. These types of illusions are called physical illusions and occur based on the way light interacts with the environment. For example, when examining a glass of water containing a straw, the straw may appear to break where the water meets the air. This happens because the refraction indexes for the 2 media are different. Another example of a physical illusion is a rainbow. Rainbows seem to be solid objects in the sky; however, they actually appear when sunlight (white light) splits into separate colors as it hits raindrops, just as a prism splits light.

After light enters the eye, 3 main types of optical illusions occur: literal illusions, physiological illusions, and cognitive illusions.

Literal illusions

Literal illusions occur when the physical object that makes up an image is different from what is perceived. For example, if you create the letter H using repetitions of the letter A, you perceive the H even though the object creating the image is actually the letter A (Fig. 1). Another example is when flowers or birds appear to form a smiley face. You perceive the smiley face, but the image is actually flowers or birds.

Figure 1  Letter H made of the letter A.

Physiological illusions

Physiological illusions are those caused by human physiology. They typically occur when parts of the eye or brain are overstimulated due to factors such as brightness, color, motion, etc.; they can also occur due to competing stimuli. An example of a physiological illusion is an afterimage. Stare at the black dot in the center of Fig. 2 for 30 seconds and then quickly look at a sheet of white paper. What do you see? This type of image occurs when looking away after staring intently at certain images or colors. When you closely observe an object for a long time, the light stimulating the retina eventually causes fatigue in the rods and cones, desensitizing that part of the retina. After looking away from the image, the less-stimulated cones in the affected area still function. The affected portion of the retina continues to produce the image for up to 30 seconds, but because the stimulation comes from only the less-fatigued cones, it is perceived as an image with complementary or negative colors.

Another example of a physiological illusion is a Hermann grid. Examine Fig. 3. How many gray dots can you count? Lateral inhibition, which occurs when an exited neuron reduces the activity of its neighbors, causes this illusion. As you look at the grid, gray dots appear to be at the intersections of the white lines, even though there is no gray dot when you look directly at the intersection.

Figure 2  Afterimage.

Figure 3  Hermann grid.

Cognitive illusions

Cognitive illusions reflect how the brain relates the image it sees to assumptions or knowledge that it already possesses. There are many cognitive illusions, and they can be broken down into 4 categories: ambiguous illusions, distortion illusions, paradox illusions, and fiction illusions.

Ambiguous illusions can be perceived in more than 1 way. An example of this type of illusion is the Necker cube. Examine the image in Fig. 4. Is the cube oriented downward and out or upward and out? Either the bottom left square or the top right square can be seen as coming toward the viewer. Perception of this type of illusion can often be adjusted at will. Another example of an ambiguous illusion is Rubin’s vase in Fig. 5. What do you see when you look at the image? The mind must determine what is foreground and what is background. If the black space is the background, you see the vase; if the white space is the background, you see 2 silhouettes.

Figure 4  Necker cube.

Figure 5  Rubin’s vase.

Distortion illusions are also sometimes referred to as geometric illusions. They occur when there is a difference between perceived distance, size, shape, etc., and what can be measured. One example is the Herring illusion in Fig. 6. Are the vertical lines straight or curved? In this illusion, the vertical lines appear curved even though they are straight. This illusion is thought to be due to the perception that one is moving forward toward the vanishing point (the center where the lines intersect). As the person viewing the illusion is not moving forward and the image is static, the mind is tricked into thinking the vertical lines are curved. Another example of this type of illusion is the Müller-Lyer illusion. Examine Fig. 7—which horizontal line is longer? The horizontal lines appear to be different lengths, but they are actually the same. This illusion plays off of typical depth cues associated with the direction of the diagonal lines at each end of the horizontal lines.

Figure 6  Herring illusion.

Figure 7  Müller-Lyer illusion.

Paradox illusions are ones that could not occur or are paradoxical. Artist M.C. Escher’s works incorporate many of these types of illusions. One of the most famous is the waterfall illusion in which water going to a waterfall appears to be coming from downhill. Another example of a paradox illusion is the Penrose, or impossible, triangle. This illusion stems from the brain’s perception that adjacent edges must join together. It’s possible to view this with other polygons; however, the image becomes distorted.

Figure 8  Penrose triangle.

With fiction illusions, one perceives something that is not actually there. An example of this type of illusion is the Kanizsa triangle. In Fig. 9, what shapes are present? The brain infers the existence of the triangle even though a triangle is not actually present.

Figure 9  Kanizsa triangle.


The next time that you or your students view something that seems a little weird, stop and ask whether what you are seeing is actually there, a trick of the light, or an optical illusion. This topic is a perennial favorite as well as an engaging subject to capture your students’ attention.