A riddle

Ok, I'll reveal the solution of the riddle.

A position-dependent flash sequence can have two reasons. The simplest one is that it's a very old airport with rotating beacons for the runway lights. All beacons rotate synchroneously, so their light beams always have the same angle. Therefore when looking at the runway sidewards from a distance, they all flash at the same time. However when an aircraft approaches the runway or takes off, the beacons are seen in slightly different directions by the pilot, so their light beams become visible in sequence.

However, our airport doesn't use rotating beacons but xenon flashes. Still, you see them flash in sequence even when they all light up at the same time. The reason is indeed the optical effect mentioned by Blattsalat. Our optical nerves react faster on a bright flash than on a dim flash, so close lights are seen a little earlier than far-away lights. Although the flashes appear to have the same brightness, it's an optical illusion - a flash close to us has in fact four times the brightness than a flash in twice the distance. For a similar reason, when you see street lamps suddenly go on in the evening, you have the impression that they are switched on in sequence.

BTW, this was a small hypothetical airport - big commercial airports use non-flashing runway lights and lights that flash in a real sequence _before_ the runway.

the phenomenon with the turning wheels or ther spinning objects apearing to turn the wrontg way only apears at a specific speed this is when the wheel has turned almost completely since the last frame of the movie and so the brain percieves it as moveing backwards, it then apears when the wheel has done almost two rotations since the last frame and so on. it is because film is a seriese of pictures whereas in real life we see the continual stream of movement so dont notice it.

has to do with strobic effect and the mechanics in how our eyes work ( we don't see continuously, but in 'frames' just like a movie or video game)

the wheel effect has to do with the "scanning" frequenz of our eyes/brain =~ 24frames a second

if the wheel turns exactly the angle of one spoke in one frame our eye cant recognize if its turning forward or backwards. similar to what happens if you post 2 screenshots next to each other where one wheel has turned just the ammount of one or a multiple of the spoke angle. (the wheels match on each other so does our brain "see" it as well)

if now the wheel turns just a little bit less then the distance of the single spokes it seams to turn backwards.

this would work with a flip-book as well or your monitor: imagine a point at the very right of the screen. Now with every frame the point moves 100% of the direction. the result would be nothing - a still standing point.
if the point now only moves 99% of this direction per frame, it will start to move slowly from the right side to the left one till it appears on the origin again.

now its up to you to figure out what happens at 10% or even at 50%

phew, way to much text for something so unimportant like this
especially if you put some spin-wheels on your car

sreehc

our eyes don't have a scanning frequency or a shutter like a film/video camera.

i think sometimes wheels themselves can work like a camera shutter. for example if you look through openings of the rims at something moving behind them or if some reflection on the chrome only happens at a certain angle which changes while overtaking the vehicle or something like that. then your eyes receive light flashes at a certain frequency and a strobe effect occurs.

its not our eyes but the brain and the frequency it needs to calculate what input it gets from the eyes.
if our brain would "see" faster we would be able to see bullets shot out of a gun.
because its damn slow we need special flare ammuniton for it

of course our vision bandwidth is limited but there is no separation into frames at a certain frequency or something like that. we see continuously and not like a camera.

also if our temporal resolution were limited to ~ 24fps we couldn't see the difference between film and video footage that easily and games wouldn't look better at 60fps but 24fps would suffice.

The point of my post, of course, was not about HOW it happens in real life, but that it CAN happen (contrary to what JCL stated).

I don´t think lifeforms see in pictures. If something moves very fast we see a blurred mix of endless sequences. Very simple example, if you watch out of the driving train you don´t see every third bush, you see a smeared green line. Or a rotating picture with black and white stripes gets, grey, acutally it doesnt gets grey it gets colorfull, but that´s still not a single picture, but a mix of endless pictures.

By the way, semimodern video games, have the choice to play them with 50Hz or 60Hz. And with 60Hz it looks indeed more realistic. My teacher always says 30 frames are enough for a prerendered movie, but I think that's is so wrong because we see the interpolated frames as well. The motion blur. I mean if something moves with 60 frames per second in on 1/6second from left outside of the screen to right outside of the screen we should recognize something hushing from left to right. With 30 frames the movement from left to right has interuptions and should be less visible. Yeah it´s moving the same fast, but with bigger interuptions the object should be more invisible.

It´s like watching somethign when a stroboskob is on isn´t as easy as without stroboskob.

from what i've heard, the frequency at which our brain interprets the image coming into our eyes is between 60 and 80 hertz. my old software design teacher told me this when i asked him for some help and he said "excuse me" and changed the settings up to 80 hertz and explained that otherwise the image was constantly flickering, from what he could see. a quick look at wikipedia doesn't seem to confirm this, but that's just what i heard.

julz

No, one reflection would not suffice for that. You need two periodic effects with slightly different frequencies. So the second one was likely a reflection from another wheel. Maybe it was the wheel of your own car that you have seen reflected in the truck's wheel. Or it was a reflection from a vibrating part of the truck.

I see this all the time actually. Seems once a car gets past 40mph the wheels seem to go backwards. I'm also very near sighted and wear contacts so this might have something to do with it.

of course there is no framerate for our visual organs (nobody said that). the fact that you see blurred hands if you move it fast enough infront of your eyes idicates the speed our brain needs to interprete the visual ("framerate" if you dont mind the word ).
i doubt this spinback effect can be seen in real life. because if the wheel turns fast enough it will start to look blurred instead of turning backwards.

the strobe effect jlc mentioned (same as the frames of a movie) is what "saves" the position of the wheel. if it fits the alias effect sheme then our brain will interprete it as backwards moving, still standing or forward motion.

in real life other optical illusion would be needed i think to achieve this effect (if possible)...something that "burns" or fakes single frames to our visual system as a tv would do (strobe lights maybe)

i also doubt it can be seen in real life (at least that easy) though i have never thought about it so far. I will have to take a look today when a car passes by

cheers

You need to be moving about the same speed as the car. Best to do this from a stand still and speed up witht he car, also best not to be driving while doing this.

actualy, there is a 'frame rate' to our vision. as you know we see by light passing to our retina to the rods and cones, which are sensitive to differnt frequencies of light ( color).

but the rods and cones do not run continuosly. but turn on and off rapidly

The rods and cones contain visual pigments. Visual pigments are much like any other pigments in that they absorb light with absorption sensitivities that are wavelength dependent. The visual pigments have a special property, however, in that when a visual pigment absorbs a photon of light it changes molecular shape and at the same time releases energy. The pigment in this changed molecular form absorbs light less well than before and thus is often said to have been bleached. The release of energy by the pigment and the change in shape of the molecule together cause the cell to fire, that is to release an electrical signal. which then travels down the optic nerve to the brain. A bleched receptor , now far less sensitive to light won't release another electrial signal ( unless hit with significatnly intense light, like a blinding light)

The melinin in our eyes ( same chemical that causes skin pigmentation based on the quantity of melinin retaind by a cell, which also determins our eye color, darker the eye color the mroe melinin your eye has) chemicaly reacts in your rods and cones to unbleach them to reset these photo receptor cells in your retina for light sensitivity. the Iris ( the colored part of your eye) acts like a polerizing filter on a camera. To filter light intensity. Again the iris color reflects the ammount of melinin in the eye, and the more melinin the more it blocks light.

( side note, Melinin naturaly is a black color, because it absorbs light ( thus why you see things black))

It's not our brain that divides vision into frames, its the mechanics of our eyes. the very basics of how our vision works which was reversed engineered to create the picture tube decades ago. Even today some of the mechanics and chemical processes arn't fully understood ( its hard to test an operating organic vision system afterall)

Thanks for the explanation. I think it makes sense that our eyes do have a limit on the amount of impulses that can be fired, otherwise we would be constantly blinded by light.

Cheers