Another speed/light question

[SC00BY]

Why sometimes, say in a motorbike race does the inside of the wheel look like its going backwards? it obviously isnt but when the bikes set off they seem to go forwards/backwards/forwards??

[Clanad]

Here's better answer than I could construct in the same space...The movie is a series of still pictures that flash on and off 24 times every second. That's plenty fast enough for your brain to think the motion is continuous because your eyes can't see the separate frames. No finite sampling rate, however, is fast enough to capture the true continuous motion.

The wagon wheels can even appear stationary: Suppose the movie camera snaps a frame and the wheel turns just enough in the 1/24th of a second before the camera snaps the next frame so that the next spoke has rotated into the position of the first spoke. Then the camera snaps another frame. The second frame looks identical to the first frame even though the individual spokes are in different positions because all the spokes look alike and they line up with the first-frame positions. At this wagon speed, the wheel doesn't seem to be turning at all in the movie.

If the wheel turns a little faster than this, the spokes seem to slowly move forward. That's because the second spoke, going a little fast, went by the position of the first spoke by the time the camera snapped the second frame. If the wheel turns a little slower, then the spokes appear to move slowly backwards.

Thus, wagon wheels can appear to move backwards, then forwards as the wagon picks up speed. If the wagon slows down, the wheels can seem to change direction again and go backwards. It all depends on how fast the camera snaps frames relative to the wagon wheel speed.

Contd.

[Clanad]

Contd.

Additionally, (stay with me here) some indviduals see the "backward spinning wagon wheels" illusion in real life � outdoors in the steady sunshine and not watching a flickering movie. Almost all people do especially if they look at the spinning wheel for a prolonged time (13 seconds to 8 minutes, in one experiment).

Neuroscientists, however, disagree on what causes the illusion we experience � some agreeing with the snapshot idea. One states, "We normally see motion, as in movies, by processing a series of visual episodes."  In other words, snapshots. Another calls it "batch-like effects in vision." Again, snapshots.

Others don�t agree with the snapshot explanation. some behavior scientists think it�s our tricked perception � a phenomenon called "perceptual rivalry" � that causes the illusion. In that case, our minds interpret the same scene in different ways � alternating between the two different interpretations...

[SC00BY]

Gee wizz, great answer thanks very much. was something i always wondered

[gen2]

The scene does not necessarily have to be filmed to see this effect.  If a rotating spoked object is viewed under fluorescent light you will see the same effect.  Some record turntables used to have (maybe still do) black and white bars round the edge so that when rotating at the correct speed, it would appear stationary under such lighting.  You can easily see the strobe effect of fluorescent lights by spreading your fingers and waving in front of a fluorescent light tube (also a TV screen or a computer monitor).

[BigDogsWang]

Similarly, when you see a computer screen on television, it usually appears to be flickering, because VDU's and TV's flicker at something like 50 times a second, and are rarely in sequence when one is watched on another. 

[LiIIabet]

our vision is nothing like a 'batch like' process nor is it 'a series of frames' nor is it ever a 'frame' at any point.