The effect is very observable in sunlight with airplane propellors. Turboprops are particularly good because they very gradually build RPM from zero so you can watch the transitions from actually seeing the blades to a blur to apparent backwards motion that seems to transition to forward motion then to a blur again. It’s rather hypnitic to watch. So much so that on aircraft carriers there has to be a human barricade around propellor planes like the E2C Hawkeye to keep people from walking right into a turning prop.
Are you sure about that? If so, it should be pretty easy to measure the frequency of sunlight. Do you know if this has been done?
I noticed this phenomenon watching my room light through an electric fan on the Medium setting, which must be pretty close to 60 Hz. The white light would slowly pulse blue and orange. It was weird.
I’ve too seen the turboprop example… standing in front of the engine, watching it spool up or down in speed, and there is a strobocopic effect…
My theory is an internal “frame rate” between our eyes and our brain… Same theory as to why you can sometimes see “individual” blades of a helicopter’s rotor blades, from the ground, looking up.
It’s also really easy to achieve - in sunlight - by spinning a bike’s front tire really fast. As it slows, it’ll pass through the correct speed where it appears to be going backward.
Are you saying you can get the stroboscopic effect without a strobing light source, or else two periodic activities (like in film)? Here’s what Cecil has to say about this sort of claim:
Ah yes I’m sure that’s exactly what Cecil had in mind in the way of “facts”. Nevertheless, I will try it. Does it have to be a bicycle tire? Can I watch cars’ hubcaps? Or use my electric fan? I’m afraid I don’t have a turboprop handy.
Thanks for the sarcasm, but this is a testable claim. What would have been a better offer? I can’t very well film this, because that would invalidate the results, nor will I drive to your and Cecil’s houses.
I know I can get it to happen with a bike tire, looking at the spokes. I didn’t bring up hubcaps or electric fans or turboprops.
Believe it or not, I don’t know the frequency at which it occurs. If it’ll help you, though, let me know how I can measure the frequency at which I spin a bike tire. The phenomenon’d probably vary by how far apart the spokes are, but we can try. I’ll tell you, I have to spin it hard, and then watch the wheel slow down.
>>but the burden of proof, it seems to me, lies with you.
There’s a stock SDMB response, appropriate for a different situation. See, I’m trying to prove it. That’s the thing. I don’t know how I could be more reasonable in my challenge, but you’re acting like I’m a troll. And you know what else? I’m not the first person in this thread to challenge it.
Padeye says you can see it in turboprops in sunlight - you asked if he meant looking between two sets of blades, but he didn’t say that, and in fact, his description seems to imply something else.
butler1850 mentions seeing helicopter blades overhead that appear to be stationary. I’m going to guess that was also in sunlight.
Finally, getting back to your sarcastic “facts” statement, that is exactly what Cecil was looking for. His explicitly stated problem with the claims was nobody could recall the circumstances. Here are three specific claims of the phenomenon in sunlight.
Alright, first of all, chill out. I don’t believe what you’re claiming yet, but I don’t think I’m being hostile, either.
Now then, here’s how you can measure the frequency. Count the number of spokes. Put some sort of marker on the edge of the bike wheel. Get it up to a constant speed, where it looks like the spokes are standing still. Using a watch, time the amount of time it takes the wheel to go around 10 times. The frequency will be:
You know I see this all the time on cars, during daylight, night time whenever. Maybe the reason that some people do see it and others do not is because of eye differences.
I never pay much attention to the details but for me I would guess that I see it most often around 50MPH and sitting right next to the car. I may pay more attention to car wheels though since as a motorcyclist that’s what I look for to see what the car intends to do.
One potential explanation, for various sorts of propellor and rotary wing: The blades are tilted to some extent, so you’ll get specular (or semi-specular) reflection to your eye only when the blade is in a particular position. So you see each blade most clearly when it’s in that position, and it looks like it’s stopped there.
I’m in the you-can’t-see-it broad daylight camp here.
However, I once saw a stroboscopic effect with a passing train. There are sets of balustrades along the upper stories of the Santa Ana trains station. While I was waiting for a train one late afternoon the sun was casting the shafoes of the balustrades down across the track in the station.
As a train passed through the alternating sunlight and shadow on the wheels of the passing train gave a noticable strobe effect. My guess is that the spacing of the shadows was near some mulitple of the wheel’s circumference, letting the light illuminate the same part of each wheel as it flashed into the sumlight.
Cecil has committed an error of ommission–he failed to do a simple literature search.
I, for one, believe Padeye, bup, and butler1850.
The “stroboscopic effect” in daylight arises not because of a trick of light reflections or because sunlight is emitted stroboscopically; more likely, it is because we may actually see at a framerate of 2-20Hz. That is, our (i.e., human) visual processing system is very similar to a movie. We essentially process static images in quick succession.
For confirmation of this “wagon wheel” illusion in broad-daylight, just try it out. Take some spoked object outside and spin it around. If you’re not an experimentalist, get your hands on a copy of
Proceedings of the National Academy of Sciences of the United States of America, 1996, vol. 93, iss 8, p 3693.
There you’ll find:
“The Wagon Wheel Illusion in Movies and Reality” by Purves, et al.
Looks like it’s online, but I’m having trouble getting to the full article:
Do they make any determination of the frequency? You said 2-20 Hz, but that’s way too broad a range to produce this effect. Or is it inconsistent? (I still do intend to try this, but I’ve been busy.)
And someone else here can confirm or deny this, but it’s quite likely that Cecil wrote the column before 1996, and if so you can’t really hold him accountable for omitting a reference that didn’t exist yet.
It’s a biological system, so it pretty much defines your view of “inconsistent”. You shouldn’t expect much precision in defining a frequency, as neuronal processes are quite complicated and will likely behave at different “rates” for different specific stimuli (i.e., a wheel with blue spokes might counter-rotate at a different speed than one with black spokes).
Are you saying you can get the stroboscopic effect without a strobing light source, or else two periodic activities (like in film)? Here’s what Cecil has to say about this sort of claim: