# Persistence of Vision **URL:** \ **Category:** Factual Questions\ **Created:** [August 16, 2025, 5:24pm UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142 "2025-08-16T17:24:39Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![kaylasdad99](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kaylasdad99/32/3398_2.png) [@kaylasdad99](https://boards.straightdope.com/u/kaylasdad99)\ **Post date:** [August 16, 2025, 5:24pm UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/1 "2025-08-16T17:24:39Z")
You know how in Sixties-era Western TV shows like _Bonanza_, when they showed a wagon wheel rotating, it would look like the wheel was going backwards (primarily due to shutter speed on the camera doing the filming, AIUI)? I just noticed a similar phenomenon with my ceiling fan. It rotates CCW (CW if you’re looking at it from the actual surface of the ceiling, which I’m not inclined to try doing). When I watch it from below, there is a bit of a blurred image, but what bugs me is this: in my eyes, the outer tip of the fan blade appears to _lead_ the rest of the blade. Intuitively, I would expect to understand the image of the tip _trailing_ the blade, much as a pennon would trail after the staff to which it was attached, when carried into battle. What gives? Is it a trick of the eye?,\* or am I misusing the concept of persistence of vision? \*ETA: (I feel uncomfortable trying to spell _trompe l’oeil_ in public.) ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [August 16, 2025, 5:30pm UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/2 "2025-08-16T17:30:41Z")
Probably caused by the illumination flickering at 120Hz. (Basically, the same as the wagon wheel phenomenon). ---
**Author:** ![Chefguy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chefguy/32/138_2.png) [@Chefguy](https://boards.straightdope.com/u/Chefguy)\ **Post date:** [August 16, 2025, 5:32pm UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/3 "2025-08-16T17:32:31Z")
Nailed it in one. ---
**Author:** ![Sigene](https://avatars.discourse-cdn.com/v4/letter/s/df788c/32.png) [@Sigene](https://boards.straightdope.com/u/Sigene)\ **Post date:** [August 16, 2025, 5:33pm UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/4 "2025-08-16T17:33:11Z")
Just a guess but your LED lights are flickering at a speed that your eyes cannot detect leading to the shutter phenomena. Why the tip appears to lead the rest of the blade is something beyond my knowledge, except for a couple of guesses. The tip of the blade is further from your flickering LED light which cause it to cast a bigger shadow then the blade body. You are perceiving the broadened shadow as part of the tip. Or maybe There are two LED lights flickering slightly out of sync and the one of them lights up the tip first before the other one lights up the body of the blade. ---
**Author:** ![running\_coach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/running_coach/32/15836_2.png) [@running\_coach](https://boards.straightdope.com/u/running_coach)\ **Post date:** [August 16, 2025, 5:35pm UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/5 "2025-08-16T17:35:31Z")
And digital video creates it’s own weirdness. Explained by @Mangetout > **[Why Do Propellors Look Bendy On Video?](https://www.youtube.com/watch?v=k4b55YJd9Ck%2F)** > > I recently flew on a prop-driven plane- I shot some video of the moving propellors out of the window - and in this video, I will explain why propellors look ... ---
**Author:** ![kaylasdad99](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kaylasdad99/32/3398_2.png) [@kaylasdad99](https://boards.straightdope.com/u/kaylasdad99)\ **Post date:** [August 16, 2025, 5:37pm UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/6 "2025-08-16T17:37:32Z")
> [@beowulff](#): > > Probably caused by the illumination flickering at 120Hz. > > (Basically, the same as the wagon wheel phenomenon). I don’t have any electric lights on as I’m posting/watching. ---
**Author:** ![Roderick\_Femm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/roderick_femm/32/14875_2.png) [@Roderick\_Femm](https://boards.straightdope.com/u/Roderick_Femm)\ **Post date:** [August 16, 2025, 5:44pm UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/7 "2025-08-16T17:44:00Z")
> [@kaylasdad99](#): > > (I feel uncomfortable trying to spell _trompe l’oeil_ in public.) Trompe l’oeil means something like “cheat the eye” and usually refers to 2D paintings that very cleverly simulate 3D in ways much more visually persuasive than mere perspective. I don’t think it has anything to do with the phenomenon you described. But I am prepared to be disabused of my opinion, since I am not an expert. ---
**Author:** ![kaylasdad99](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kaylasdad99/32/3398_2.png) [@kaylasdad99](https://boards.straightdope.com/u/kaylasdad99)\ **Post date:** [August 16, 2025, 5:54pm UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/8 "2025-08-16T17:54:50Z")
Maybe I’m thinking of pareidolia. ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [August 16, 2025, 6:03pm UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/9 "2025-08-16T18:03:47Z")
> [@Roderick\_Femm](#): > > Trompe l’oeil means something like “cheat the eye” and usually refers to 2D paintings that very cleverly simulate 3D That is absolutely correct. I once drew a cartoon I titled [Trump L’Oeil](https://schumanchu.eu/html/l/l_00060trumploeil.html) to play with this concept. It’s all so long ago now… I have a ceiling fan myself, about 160 cm in diameter, three blades, dark tinted balsa wood, with several speeds. I cannot see the phenomenon described by the OP, I have no idea what could cause such an illusion. The blades don’t bend neither forwards nor backwards at any speed. At high speeds they become very blurred and hard to see. That is persistance of vision, I believe: there is still brown impression in my retina when the white from the ceiling is coming through. But I perceive no shape change. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [August 17, 2025, 3:03am UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/10 "2025-08-17T03:03:19Z")
Wild guess. Maybe the visual processing is latching onto the outside edge of the end of the blade and that is allowing faster processing of the location of the end. Our visual system keys on edges and contrasts rather than extents of something. Something that explains quite a few optical illusions and phenomena. You can think of processing as a hierarchy of steps with very basic edge detection at the top, and thence feeding into more specialised layers that extract further information as data flows down. It you confuse the top layers, there is no going back deeper down. ---
**Author:** ![Dereknocue67](https://avatars.discourse-cdn.com/v4/letter/d/6de8d8/32.png) [@Dereknocue67](https://boards.straightdope.com/u/Dereknocue67)\ **Post date:** [August 17, 2025, 7:44am UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/11 "2025-08-17T07:44:19Z")
It’s my understanding, this is called the stroboscopic effect. It’s an illusion that can happen when the object’s rotation speed is close to a multiple of the frame rate of the video camera _or_ the flickering frequency of the light source. ---
**Author:** ![susan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/susan/32/17537_2.png) [@susan](https://boards.straightdope.com/u/susan)\ **Post date:** [August 17, 2025, 10:40am UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/12 "2025-08-17T10:40:58Z")
That’s more like seeing words in the trees or Jesus on your burnt toast. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [August 17, 2025, 12:47pm UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/13 "2025-08-17T12:47:24Z")
> [@beowulff](#): > > Probably caused by the illumination flickering at 120Hz. For $17 you can buy a [Stroboscope Disc](https://www.amazon.com/Turntable-Strobe-Disc-Stroboscope-Tests/dp/B01NAX38FV) to check the speed of your turntable. It uses the 120 Hz flicker from room lighting. I recall it worked much better with fluorescent vs incandescent. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 17, 2025, 3:43pm UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/14 "2025-08-17T15:43:52Z")
Yeah, incandescent bulbs take longer than 1/120 second to cool down, so they keep a more or less steady light output over the whole AC cycle. ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [August 17, 2025, 6:28pm UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/15 "2025-08-17T18:28:26Z")
The OP said in #6 that they are not using any electric lights when they observe this phenomenon. (I presume that they’re viewing the fan in sunlight.) And no camera is involved. So it can’t be a stroboscopic effect. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [August 18, 2025, 7:10pm UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/16 "2025-08-18T19:10:57Z")
Yeah, there’s a trick where you can make a straight object like a pencil appear to be floppy and flexible by wobbling it in a particular way, which might be a related phenomenon ---
**Author:** ![thelurkinghorror](https://avatars.discourse-cdn.com/v4/letter/t/7c8e57/32.png) [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)\ **Post date:** [August 20, 2025, 4:06am UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/17 "2025-08-20T04:06:15Z")
Here’s an online demo with scientific explanations > **[Wagon-wheel effect](https://michaelbach.de/ot/mot-wagonWheel/index.html)** > > Demonstration of the “Wagon-wheel effect” ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [August 20, 2025, 2:58pm UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/18 "2025-08-20T14:58:53Z")
> [@kaylasdad99](#): > > primarily due to shutter speed on the camera doing the filming Just to be clear, the shutter speed is how long the shutter stays open for each frame. The thing that causes the wagon wheel effect is the frame rate–how many frames are taken each second. A little bit of a tangent but here’s a really interesting example. [https://www.youtube.com/watch?v=VXJ0u3ZNdNg](https://www.youtube.com/watch?v=VXJ0u3ZNdNg) ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [September 19, 2025, 2:14am UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/19 "2025-09-19T02:14:09Z")
Some of the people reading this thread may be seeing “suggested” topics at the bottom of the page. You will get a complete different set of suggestions if you switch to “related” topics. The human eye/brain has it’s own “flicker effect” at around 13Hz. You don’t need a camera or strobe light. [The Continuous Wagon Wheel Illusion Is Associated with Changes in Electroencephalogram Power at ∼13 Hz | Journal of Neuroscience](https://www.jneurosci.org/content/26/2/502) ---
**Author:** ![pjd](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pjd/32/3009_2.png) [@pjd](https://boards.straightdope.com/u/pjd)\ **Post date:** [September 19, 2025, 9:52am UTC](https://boards.straightdope.com/t/persistence-of-vision/1022142/20 "2025-09-19T09:52:30Z")
I notice that the drive shaft between the rotor blades and the chopper’s body can be seen rotating .. how does that work ? - is that rotating at a different speed ? [Next page](https://boards.straightdope.com/t/persistence-of-vision/1022142.md?page=2)