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.)
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.
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.
That is absolutely correct. I once drew a cartoon I titled Trump L’Oeil 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.
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.
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.
For $17 you can buy a Stroboscope Disc 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.
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.
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