The apparent color of a rapidly spinning wheel or disk is an interesting phenomenon in physics and visual perception. When a wheel spins fast enough, it can appear to change color or become blurry due to the limitations and quirks of human vision. Let’s take a closer look at why this happens.
How the Eye and Brain Process Color and Motion
To understand the spinning wheel illusion, we first need to understand some basics about how the human eye and brain process color and motion information. There are two main factors at play:
- Persistence of vision – The eye and brain retain an image for a fraction of a second after it has disappeared. This is what allows us to perceive motion in movies and animations.
- Color processing – The eye has receptors called cones that are specialized to detect different color wavelengths. The brain combines signals from the different cones to perceive color.
Normally, these two functions work together seamlessly. But when an object is moving extremely fast, persistence of vision causes colors to smear and blend together, creating an optical illusion.
Persistence of Vision and Motion Blur
Persistence of vision refers to the fact that the eye and brain briefly retain a visual stimulus even after it has disappeared. This afterimage lasts for approximately 1/20th to 1/10th of a second.[1] This mechanism is key to our ability to perceive motion. When a sequence of still images is shown at a fast enough rate (as in a film reel), the afterimage of one frame lingers while the next frame appears, creating an illusion of continuous movement.
However, persistence of vision can also cause a downside – motion blur. When an object moves quickly across our field of vision, the lingering afterimage blurs together with the new position of the object. This makes rapidly moving objects appear indistinct or smeared.
The table below shows approximate speeds at which different levels of motion blur become noticeable:
| Speed | Motion Blur Effect |
|---|---|
| 10°/sec | No noticeable blurring |
| 40°/sec | Blur just noticeable |
| 80°/sec | Blur clearly visible |
| 160°/sec | Details becoming unrecognizable |
As the table shows, blurring starts becoming evident around 40°/sec as persistence of vision causes our perception to smear. At very high speeds of ~160°/sec, the details of a moving object can become completely unrecognizable due to heavy motion blurring.
Color Processing in the Eye and Brain
In addition to motion blur, the way our eyes and brain process color information also plays a role in the spinning wheel illusion.
The retina contains two types of photoreceptors – rods and cones. The cones are specialized cells that allow us to perceive color. There are three types of cones tuned to different wavelengths of light corresponding to red, green and blue color ranges.[2] The brain combines and contrasts signals from the three cone types to generate our perceptual experience of different colors.
Importantly, the red and green cones have a slower response time compared to the blue cones. The blue cones react quickly to changes in color, while the red and green take longer to respond. This lag in responsiveness will become important when we look at fast spinning colored wheels.
What Happens When a Wheel Spins Rapidly
Now let’s examine what happens when a wheel spins fast enough to cause substantial motion blurring. Due to persistence of vision, the color will smear from the wheel’s current position towards wherever it was a fraction of a second earlier. The blurred color trail creates an additive mixture of colors that shift along the wheel’s rotational path.
Consider a wheel with alternating red and green segments. When stationary or slowly spinning, we simply perceive distinct red and green sections. But at moderate speeds around 60 rpm, a subtle yellowish fringe starts becoming visible between the red and green due to color mixing. As the speed increases further, the main color visible shifts closer to yellow as the red and green blur together more extensively. Around 200-300 rpm the wheel may appear yellowish with faint hints of the original colors.
Additionally, recall the difference in response times of the red/green and blue retinal cones. The lag in red/green processing means these colors persist slightly longer in our vision. So when a multicolored wheel spins quickly, reds and greens dominate over blues in the blurred mix of colors. This intensifies the shift toward yellow in a spinning wheel with multiple hues.
Other Factors That Influence the Illusion
A few other physical and perceptual factors can influence the appearance of a rapidly spinning wheel.
- Wheel size – Larger diameter wheels require higher speeds to generate the same angular motion blur. A larger wheel needs a higher rpm to appear distorted.
- Distance – Standing nearer to the wheel increases the angular velocity perceived by the eye, enhancing the illusion at a given rpm.
- Surroundings – A wheel against a solid black or white background shows the clearest color change. Cluttered backgrounds distort the effect.
- Direction – The direction of spin and shape of color segments also impact the mixed hues.
So in summary, the factors that control the color blend are the wheel’s absolute rotational speed, the observer’s distance, the wheel’s contrast against the background, and the orientation of the colored segments relative to the spin direction.
Examples of Spinning Wheel Color Illusions
The spinning wheel or “color wheel” illusion has been applied creatively in many settings. Here are some artistic examples that intentionally leverage the effect:
- Kinetic sculptures – Spinning sculptures with precisely arranged color segments produce captivating illusory blends.
- Color wheels – Simple paper or digital color wheels clearly demonstrate the principle for educational physics demos.
- Magic tricks/toys – Cheap children’s toys use cheap spinning plastic discs to “magically” change displayed colors.
- Hypnosis wheels – High contrast patterns are designed to create disorienting optical flicker effects.
- TV test cards – Spinning patterns allow TV engineers to optimize color balance adjustments.
These examples creatively leverage the spinning wheel illusion, producing novel visual effects or serving practical applications in entertainment, art, education and technology.
The Illusion and Color Theory
The spinning color wheel illusion also relates closely to principles of color theory and additive light mixing. Let’s look at some of these connections:
Additive vs Subtractive Color
There are two main ways to mix colors of light – additive and subtractive mixing:
- Additive – Combining wavelengths of colored light. Red + Green = Yellow. This is the principle behind TV/computer screens.
- Subtractive – Absorbing certain wavelengths using pigments. Yellow paint absorbs blue light but reflects red+green. This is how mixing paints works.
The color wheel illusion demonstrates additive light mixing. The blurred color trail is an optical mix of wavelengths, not due to chemical pigments. This causes the shifting hues as speeds increase.
Primary Colors
In color theory, primary colors are those which can be additively mixed to create other hues. For light the primaries are red, green and blue – hence RGB color systems. This relates to the three types of retinal cones tuned to those wavelengths.
The dominance of red/green in the spinning wheel effect ties directly to the cone responsiveness. The changing appearance effectively mixes different proportions of the visual primary colors.
Color Opponency
An additional part of color vision is called color opponency. This refers to our perception comparing and opposing color signals from the retinal cones. One such “opponent channel” contrasts red vs green hues.
The yellow appearance of a spinning red/green wheel results partly from the red-green opponent system becoming overwhelmed and blending the two colors together. This demonstrates a form of “opponent color breakup”.
The Illusion and Refresh Rate Effects
Closely related to the spinning color wheel is the flicker seen on TV/computer screens with low refresh rates. On early TVs and CRT monitors, the 50-60 Hz image refresh would sometimes make moving images appear disjointed or flickering. This relates to the same principle of persistence of vision interacting with screen redraw rates.
Higher resolution cameras also require higher frame rates to avoid perception of flicker. Again, once the frame rate exceeds the ~50 Hz limit of visual persistence, the motion appears continuous rather than flickering. So increasing scan/refresh rates helps cameras and monitors avoid visual artifacts related to the same brain mechanisms as the spinning wheel effect.
Scientific Explanations and Theories
Over the centuries, various theories and models have been proposed to explain the spinning wheel illusion, dating back to Newton.[3] Some key perspectives include:
- Newton – Suggested colors mix additively as rate exceeds processing ability.
- Fechner – Proposed colors blend via “sensation curves” and summation.
- Plateau – Dominant blur-based theory of persistance causing smear.
- Benham – Latency difference between processing of different hues.
Overall the modern explanation involves a combination of the blurring effects of visual persistence, and the variable latency in color processing by the retina/brain. So in summary, it stems from:
- Persistence of vision causing motion blur.
- Differing response times of the retinal cone cells.
- Color opponency mechanisms becoming overloaded.
Together these factors culminate in a variable perceived color determined mainly by speed, geometry and contrast. The illusion effectively demonstrates key aspects of visual neuroscience and color theory in action.
Practical Applications
Understanding the spinning color wheel phenomenon allows creating useful applications that leverage the effect:
Entertainment
Spinning color wheels are used in toys, novelty items, decoration and magic tricks. The visually striking effects lend themselves well to entertainment applications.
Psychology/Neuroscience
Carefully controlled color wheel experiments help reveal aspects of human visual processing. The effects shed light on neural mechanisms of motion perception, color interpretation and visual persistence.
Color Mixing
Rapidly blending transitional hues from color wheels allows smoothly shifting between any color combination. This can assist in color mixing calculations and animations.
Calibration
TV test cards use spinning color wheels to evaluate and calibrate display settings like contrast, color balance and refresh rates.
Conclusion
In summary, the illusory color change of rapidly spinning wheels stems from a combination of motion blur from visual persistence, and the variable response times of retinal color receptors. This results in a smeared blend of hues that shifts toward additive mixtures of certain primary colors in the wheel. The effect is an interesting phenomenon that reveals key aspects of visual processing and color perception in the human brain.
Understanding the mechanism behind the spinning color wheel illusion allows creatively leveraging it for novel effects and applications. As a compelling visual effect with roots in the very nature of human vision, we are likely to see uses of spinning color wheels persisting well into the future.
References
- Schwartz, S. H. (2009). Visual perception: A clinical orientation. McGraw Hill Professional.
- Backhaus, W., Kliegl, R., & Werner, J. S. (1998). Color vision: perspectives from different disciplines. Walter de Gruyter.
- Howard, I. P., & Rogers, B. J. (2002). Seeing in depth, volume 2: Depth perception. I Porte Books.