10 Cool Optical Illusions and How They Work

The Hermann Grid Illusion

Sometimes we see things that aren't there, and the Hermann Grid illusion is a great example. Notice how the dots at the center of each intersection seem to shift between white and gray?

Like many optical illusions, different theories have been proposed to explain exactly why this happens.

• Lateral inhibition is often used to explain the Hermann grid illusion, but more recent evidence suggests that this might not be why the illusion happens. This theory suggests that the brightest at the intersections forces retinal cells to adjust the intensity. Lateral inhibition happens when the excitation of surrounding neurons inhibits a neuron's response to a stimulus.
• S1 simple-cell theory may be more helpful for understanding the illusion. Evidence supporting this theory includes the fact that the illusion is not dependent upon the size of the grid and that the illusion still occurs when the contrast of the image is reversed. S1 simple-cell theory proposes that the illusion is caused by how S1-type simple cells in the primary visual cortex respond to certain visual stimuli.

The Spinning Dancer Illusion

The popular illusion made the rounds on blogs and websites a few years ago, supposedly as a test to determine if you are "left-brained or right-brained." In reality, the illusion occurs because our brains must attempt to construct space around the spinning figure.

Researchers suggest that the illusion stems from bistable perception, where the ambiguous two-dimensional figure is perceived from two differing perspectives. Since there is no third-dimension to provide important perceptual information, the brain attempts to fill in the space around the spinning figure.

The Ames Room Illusion

Would you be surprised to learn that the two people in the image at the left are actually the same size? Known as the Ames room illusion, this visual trick has been utilized in films such as The Lord of the Rings.

The Ames room illusion works due to the trapezoidal shape of the room. From the viewer's perspective, it looks like a square room, leading to the size distortion. The small figure is, in reality, standing at a greater distance than the larger figure.

The Ponzo Illusion

When you look off into the distance, objects seem closer together as they become further away. For example, the outside borders of a road or railroad appear to converge as they recede into the distance.

The Ponzo illusion involves placing two lines over an illustration of a railroad track. Which line is longer? In reality, they are exactly the same length.

This illusion occurs because people view the scene from a linear perspective. Since the vertical lines look like they are getting closer together, the viewer interprets the top line as further off in the distance.

The Zollner Illusion

Sometimes the background of an image can interfere with how your brain interprets the image itself, as is the case with the Zollner illusion. This is one illusion that can make a viewer start to feel slightly queasy if you stare at it for too long!

This may happen due to the shorter lines' angle compared to the long lines. This causes the brain to perceive depth where there is none.

The Kanizsa Triangle Illusion

The Kanizsa triangle is a visual illusion in which the viewer perceives a triangle that does not exist in the image. It is created by the brain filling in gaps in order to perceive a more complete whole.

According to the Gestalt law of closure, we tend to see objects that are close together as a related group. In the case of the Kanizsa Triangle, we even see contour lines that don't exist and ignore gaps to form a cohesive image.

The Muller-Lyer Illusion

Here's a classic illusion that still manages to stump a lot of people. Which line is longer? In reality, both lines are the same length.

This phenomenon is known as the Muller-Lyer Illusion. A few different explanations have been proposed to explain how it works. One theory suggests that the brain misapplies information about size consistency and scaling. This ability allows people to judge size in relation to distance, but when applied to a two-dimensional field, it leads to errors.

Another theory proposes that depth cues related to the direction of the line shafts lead to misperceptions about the length of the lines. Depending on whether the shafts are facing inward or outward, the overall length of the figure may also contribute to interpretations of line length.

The Moon Illusion

If you've ever spent any time gazing up at the night sky, then you've probably noticed the moon illusion, in which the moon looks bigger on the horizon than it does higher up in the sky. Why does this happen?

Many theories have been proposed, although there is no universally agreed-upon explanation. Judgments of distance can be influenced by how the moon appears in relation to figures in the foreground. Other factors can also influence the illusion, including the color of the moon and the presence of atmospheric haze.

The Lilac Chaser Illusion

In the lilac chaser illusion, the viewer observes several different visual effects over the span of about 30 seconds. First described in 2005, the illusion is caused by a number of different factors, including negative afterimages and what is known as Troxler fading.

An afterimage can occur when you stare at something for a long period of time. When you move your eyes or the object suddenly disappears, you may briefly continue to see a version of the image.

Troxler fading is a phenomenon in which objects in your peripheral vision become blurred and disappear from vision if you start at a fixed spot for a period of time.

The Negative Photo Illusion

The negative photo illusion is an interesting example of how negative afterimages can produce a startling visual illusion. In the negative photo illusion, your brain and visual system essentially take a negative image and turn it into a full-color photo.

You can experience this illusion by staring at the image of the face for 30 seconds to a minute. Then shift your focus to look at the X at the center of the blank white image and blink several times.

The illusion works because photoreceptors in the eye become overstimulated and fatigued from staring at the image. Because they lose sensitivity, you end up seeing negative afterimage briefly when you move your eyes.