The reverse rotation effect, sometimes called the wagon-wheel effect, is an optical illusion in which a spoked wheel appears to rotate backwards. An optical illusion is any illusion that deceives the human visual system into perceiving something that is not present or incorrectly perceiving what is present. ... A spoke is one of some number of rods radiating from the center of a wheel (the hub where the axle connects), connecting the hub with the round traction surface. ... A wheel is a circular object that, together with an axle, allows low friction in motion, by rolling. ...

Reverse rotation of wheels usually only occurs in film recording (for television or cinema). It usually reflects temporal aliasing caused by a temporally intermittent display or recording system. A movie camera typically operates at 24 frames per second, while standard television operates at 29.97 or 25 frames per second. If a wheel is turning at some multiple of these frames per second, the spokes will appear to be in the exact same position every time the shutter opens. The wheel will appear to be motionless or nearly motionless (though possibly blurred) upon viewing the film. If the wheel begins to slow down slightly, it does not have time to rotate to its original position before the shutter opens again and in effect appears to be rotating backwards. Film refers to the celluloid media on which movies are printed Film is a term that encompasses motion pictures as individual projects, as well as the field in general. ... Temporal aliasing is the technical term for a phenomenon also known as the stroboscopic effect or the wagon-wheel effect. ...

To understand this, imagine that the true motion of a four-spoke wheel is clockwise. The first frame of the movie may occur when one spoke is at 12 o'clock. If by the time the next frame is exposed, the spoke previously at 9-o'clock has moved into the 12-o'clock position, then the movie will show the wheel to be stationary. If by the time the second frame is exposed, the next spoke has moved to the 11:30 position, then the wheel will be perceived to be rotating backwards. The motion perception mechanism of the human visual system has mistakenly paired the first spoke with a different one on the second frame, because this other spoke is the closest spoke that appears on the second frame. Clockwise can refer to: Clockwise and counterclockwise Clockwise (movie) This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ...

The same effect can be noted on cinematographic recordings of helicopter rotors and aircraft propellers. A helicopter is an aircraft which is lifted and propelled by one or more large horizontal rotors (propellers). ... An aircraft is any machine capable of atmospheric flight. ... This article needs a complete rewrite for the reasons listed on the talk page. ...

Reverse rotation effect is also visible on spoked wheels illuminated by lighting that is temporally modulated. When powered by alternating current, gas discharge lamps (such as neon, mercury vapor, sodium vapor and fluorescent tubes) flicker at twice the frequency of the power line (for example 120 times per second on a 60 cycle line). In each cycle of current the power peaks twice (once with positive voltage and once with negative voltage) and twice goes to zero, and the light output varies accordingly. Although 120 flickers per second is too fast for most humans to perceive directly, such lamps can nonetheless produce stroboscopic effects. It has been suggested that this article or section be merged with :AC power. ... Gas (actually, as), the GNU assembler, is the default GCC back-end. ... For the Linux, Apache, MySQL, [Perl, PHP, or Python] (a popular web server combination), see the article at LAMP. A lamp in general is any device that produces light. ... General Name, Symbol, Number neon, Ne, 10 Chemical series noble gases Group, Period, Block 18, 2, p Appearance colorless Atomic mass 20. ... General Name, Symbol, Number mercury, Hg, 80 Chemical series transition metals Group, Period, Block 12, 6, d Appearance silvery white Atomic mass 200. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... Fluorescence induced by exposure to ultraviolet light in vials containing various sized cadmium selenide (CdSe) quantum dots. ... Sine waves of various frequencies; the lower waves have higher frequencies than those above. ... This article may be too technical for most readers to understand. ...

Reverse rotations perceived with continuous displays

Purves, Paydarfar, and Andrews (1996) reported that occasionally observers reported the wagon-wheel effect under continuous illumination. They concluded that this was evidence that human visual perception takes a series of still frames of the visual scene and that movement is perceived much like a movie. Kline, Holcombe, and Eagleman (2004) confirmed the observation, but showed that the incidences of reversed rotation were independent in different parts of the visual field. This is inconsistent with discrete sampling by the visual system across the whole visual field. Visual perception is one of the senses, consisting of the ability to detect light and interpret (see) it as the perception known as sight or naked eye vision. ...

The article by Kline, Holcombe, and Eagleman (2004) is worth being considered also for another reason, that is, they were the first who reported that this illusion occurs after many seconds (or even minutes) of gaze-fixation is needed before one can see the illusion. This finding suggests that some adaptation mechanism underlies this phenomenon.

The reason for the phenomenon in continuous light is now very controversial. Kline et al. (2004) suggested that within the human visual system, extensive adaptation of motion detectors responsive to the correct direction may allow spuriously-activated detectors tuned for the reverse direction to occasionally drive perception.

References

• Kline K, Holcombe AO, Eagleman DM (2004). Illusory motion reversal is caused by rivalry, not by perceptual snapshots of the visual field. Vision Research 44 (23): 2653-8. PMID 15358060
• Purves D, Paydarfar JA, Andrews TJ (1996). The wagon wheel illusion in movies and reality. Proceedings of the National Academy of Sciences 93 (8): 3693-7. PMID 8622999