Quote:
Originally Posted by Moose
Quote:
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Originally Posted by Jeff Root
In the wagon wheel illusion, you identify each spoke in one frame
with a different spoke in the next frame.
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Yes, and what, exactly, causes the perception of "frames" in the
wagon wheel illusion when you see it happen in real life?
If eyes refreshed literally instantly, you wouldn't lose track of the
spokes of the wagon wheel, nor would you from a physical rotating
grid like the one the image is emulating. I suspect a physical grid
would preserve a similar effect, if it were rotating in almost-time
with your eyes. |
There were at least two threads in General Science discussing this.
I gave what I am satisfied is the correct explanation in one of them.
I have only ever seen actual turning wagon wheels or the illusion
on TV, in which the illusion is caused by the strobing frames of the
TV picture, in exactly the same way that it is produced by strobing
frames of the animations in this thread.
I have seen the effect in "real life" that I think you are talking about
on car hubcaps and a ceiling fan. It is essentially the same as the
wagon wheel illusion, but the strobing is provided by reflections from
shiny parts of the rotating objects. This was made clear by looking
at the slowly-rotating ceiling fan under the right lighting conditions.
If the rotating object is shiny and has repeating shapes around its
circumference, like spokes on a wheel or blades on a fan, light from
one direction, such as direct sunlight or a lamp, can be reflected off
a portion of that shape each time it is in the same position relative
to the light source and your eye. If you, the rotating object, or the
light source are moving linearly relative to the other two, the position
of the rotating part which reflects the light to your eye will change.
That can make the rotation appear to be backward.
I watched a ceiling fan when sunlight was shining on a narrow sidewalk
just outside the windows of the room with the fan. The sunlight was
reflected from the sidewalk through the window onto the underside
of the fan blades. When the blades were in the right position, a line
of sunlight was then reflected to my eyes from the shiny surfaces of
the blades. This appeared as a relatively bright flash. The rest of
the time, the blades were only dimly lit by the ambient light in the
room. As long as I stayed in one position, the light was reflected
from the blades every time they were in the same position. Let's say
it was when the blades passed through the 2-o'clock position. If I
moved over a ways, I would see the flash in the 3-o'clock position.
If I walked past the fan, I would see the position of the reflection
rotate forward or back, just like the wagon wheel illusion.
-- Jeff, in Minneapolis