This one may go down in the books of the bleeding obvious, but here goes ...
What is the name for visual effect that occurs when some circular marked object (e.g. a car wheel) spins quickly to make it appear as if the wheel (or the markings on it) is/are rotating backwards but far more slowly? And in what relation is the slower backwards motion to the 'true' forward motion, btw.
I don't know the term and the mathematical relationship, but that slower movement backwards can be mermerizing. I'll look forward to finding out the answers.
It's an example of sampling error. You'll see it typically in movies where you have a sampling rate of 24 samples per second. The rotational speed at which the wheel appears to be going backwards is going to depend on that sampling rate and on the number of things on the wheel (typically spokes on the wheel of a wagon or stage coach) that give you the detail to see the apparent rotation. If the wheel has six spokes and goes around almost one time or almost 5/6th or almost 1-1/6th of a full rotation or variations on this (e.g., almost 3-1/2 times) it will appear that it has gone backwards just that little bit less than whatever multiple of 1/6th of a rotation it did in reality. Watch the wheels as they are picking up speed. The sampling rate in TV is thirty samples per second.
Confused yet?
... or does that only apply to planets?
what a great question!
doesn't this happen in real life, too, though? (with respect to Fald's reply) seems to me I've seen it while traveling. do our eyes "see" at a certain rate?
as far as a term goes: if there isn't one already, let's make one. how about:
revolucity
... or does that only apply to planets?don't forget music.
doesn't this happen in real life, too, though?
You'll see this sort of thing if there is something causing a strobe effect. If you wave your hands, fingers splayed, in front of a TV screen you see something like it or if your main light source is a fluorescent light, which strobes at 60 times per second you'll see this sort of thing with rotating objects. Another source of this sort of strobing can be from something like fence pickets moving between you and the wheel.
Our eyes see pretty much continuously but there is a persistence of vision that lets us smooth out the strobing of movie and TV images and fluorescent lights. It becomes obvious only when we see the apparent retrograde motion of things like rotating wheels.
BY:
Since there's a word for everything else, it stands to reason there's a word for the optical illusion you described. Later today I am going to call the U of Colorado psych department and ask them if they have a name for it.
Someone below said the human eye sees more or less continuously. Somewhere deep in the recesses of my mind is a memory of a professor's telling us in class that the eye does actually sample a certain number of times a second. This was in conjunction with an explanation of how motion pictures look as though there is true "fluid" motion.
I do know that when I am driving or riding in a car I can sometimes get a stop motion of the wheel on a car beside me if I glance over at it quickly. Never a good enough picture to tell how many spokes, but enough information to be able to say that the wheel is spoked as opposed to a disc. I've always thought that I had to have taken that little snapshot in something around a 20th of a second to be able to get that much detail.
If I find out anything I'll post back.
TEd
I think it's one manifestation of what is nowadays technically called "aliasing," pronounced as in "forming an alias."
It happens when two wave phenomena are slightly out of phase (or, more likely, some integral multiple of one is slightly out of phase with the other), giving the result that one moves slightly ahead of or behind the other. That gives the illusion that a third object is present, with different properties/frequencies.
Any kind of wave will do: light (as in this thread), sound (think of tuning a stringed instrument using "beats"), radio waves. (I suspect anything based on interferometry works on this principle, but now I'm getting a little off base.)
Aliasing in medicine is seen in ultrasound imaging and must be tuned out, or at least compensated for.
Back about 1934 or 35, at my older brother's graduation from MIT, Vannevar Bush had
a display of the stroboscope, which could produce extremely frequent very powerful
flashes, the ratie of which could be changed to show and airplane propeller apparently
slowing down and stopping, so that any cracks started to develop in propeller could
be seen. Having seen this, when I see in movies on TV the wheels appearing to be
going backwards when the car is going forward, I think of it as a stroboscopic effect
I don't know what an engineer would call it.
Here is a URL that seems to agree with me:
http://mmd.foxtail.com/Archives/Digests/199911/1999.11.30.05.html
good thought, Bill! makes me think of Harold Edgerton and his strobe photos. cool stuff.
used to use a strobotuner(every band room used to have one) to tune the old trombone...
Good point. I think you've got it.
Now let's turn it from a word post into a Word post: sounds like Greek - what does "strobos" mean, anyway?
e American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
stroboscope
SYLLABICATION:
stro·bo·scope
PRONUNCIATION:
strb-skp
NOUN:
Any of various instruments used to observe moving objects by making them
appear stationary, especially with pulsed illumination or mechanical devices that
intermittently interrupt observation.
ETYMOLOGY:
Greek strobos, a whirling; see streb(h)- in Appendix I + –scope.
OTHER FORMS:
strobo·scopic (-skpk) —ADJECTIVE
strobo·scopi·cal·ly —ADVERB
wofa--I am i-m-pressed, all over the place! Tedster, I think you're right--though I am treading on very thin ice, in amongst all these medical professionals: I too seem to recall being taught that our eyes see "samples", and our brains fill in the blanks. I also believe we've had this topic (both, actually) before, but can't think of any efficient way to Search for it {them}.)
> I too seem to recall being taught that our eyes see "samples", and our brains fill in the blanks
I too might tend toward this explanation. I know that we only see very clearly in the small central portion of our field of view and fill in the rest beyond the divide of our optical nerve.
Re. Strobes ... I was once told a story about a friendly old gentleman on a train. The man said that in the 20s or 30s he had sat outside a university in Switzerland and watched a queer bloke looking at a water fountain through his hand. Intrigued he asked the man what he was up to. He was told to notice that when one opens and closes one's fingers quickly while peering through them that the foutain seemed to stop moving. Shortly after the man was greeted by the person he was waiting for and left. At the same time a friend walked up and asked 'Did you know who that was?'. He was a bit bewildered to discover he'd been chatting to Albert Einstein :-)
Anyway, thanks for your explanation Faldage, but I'm not sure about the 'error' part - it looks right to me! I'll be interested to hear your friend's comments, Ted.
by, if you are concerned about Faldage's use of the term "sampling error" be assured that he is correct. In sampling anything, there is a requirement that to prevent aliasing you must sample at a frequency beyond the "Nyquist frequency", which is twice the frequency of the maximum frequency you are interested in accurately reproducing. If there are frequency components higher than that in the process you are sampling, then they are "aliased" and it is indeed referred to as "sampling error". Anyway, with the wheel turning much faster (more than half as fast) than the movie frames are going by, you experience a real-life version of sampling error. I was trying to draw a nice graph on Matlab yesterday to help explain this but was sidetracked by "real" work.
A non-graphical example would be the sampling rate of CDs. The human ear is sensitive to sounds (on average) between 20 Hz and 20 000 Hz (20 kHz). So if I want to accurately reproduce the highest frequencies I can hear - the 20 000 Hz ones - I must sample at least twice as fast as that. So CDs are typically sampled at about 44 000 Hz (I think it might be 44 100), or there's another standard that calls for about 48 000 Hz. Both are twice as large as 20 000 Hz, so aliasing of audible frequencies shouldn't happen.
DISCLAIMER: Please don't start arguing with me about the sound of CDs and analog and why one is better than the other. I was just using the CD as a sampling-theory example.
Thanks for your thorough description, Bean. I was more poking fun than anything else. As I see it this 'sampling error' is a great visualisation of the recapitulating, reciprocating, and symetrically duplicating process indicative to the nature of perception. I think that third object wofa mentioned does exist, you know[g]. But that's just me.
I hope you get that graph finished soon, btw!
From "engines" episode # 1183
"A stream of water flowed through the Leda scene.
And in the years just before he painted her,
Leonardo began his studies of moving water. Fluid
flow scholars call Leonardo the first student of
turbulent eddies. His drawings capture that motion
so well his vision must've been stroboscopic. The
water is more than just realistic. It seems alive. And
Leonardo later went from the study of moving water
to the study of blood flow in living beings. "
I ran across this a few days ago and bookmarked it since it bears on this discussion:
http://www.yorku.ca/eye/balls.htm
Ted, what a fun site--thank you. This is from it, also; I found it under Fun Things.
http://www.yorku.ca/eye/necker.htm