What to observe
When this page has loaded, the neighboring demo is in “auto run” mode: the wheel rotation smoothly varies between standing still and a maximum speed of 100 rotations per minute (100 rpm, indicated at top right). The wheel rotation, however, looks quite different: it seems to slow down to standstill when the maximum speed of 100 rpm is indicated! It helps to know that the wheel is rendered with 20 frames per second.
What to do
With the “thumbs” of the two vertical sliders you can set the speed, the right slider is for coarse adjustment. When you begin to adjust the speed yourself, “auto run” switches off. Now you can observe that increasing the rotation speed, starting from zero, indeed initially speeds up the wheel, but then it begins to slow down! Find the speed (≈100 rpm) where the spokes stand still (the left slider helps with fine adjustment) – but observe that the centre, and certain cute irregularities around the wheel still move. This wobble tells you that the wheel still rotates, but because of the speed of 20 frames per second the spokes seemingly stand still, and can even go backwards if you speed up! There are, necessarily, additional standstills at higher speeds, here you can check this around 200 rpm.
This is what you may have observed in movies (especially in Westerns with prominent spoked wagon wheels). It should not occur in direct observation under steady daylight – though there is a little discussion in that respect, some people report this effect in daylight. Before suggesting interaction with “brain rhythms”, I have two explanatory suggestions:
- The lighting is not really steady, but somewhat rhythmic, e.g. modulated by a fence with regular gaps.
- Roads, especially when coated with concrete, usually have “tining” (grooves cut into the concrete surface a mm or so apart for traction and increased grip when wet). This tine texture causes vibration, which in combination with certain spoke shapes leads to the wagon whel effect.
- During rapid eye movements (saccades) the conditions on the retina can easily be such to briefly image stationary spokes.
For this effect to occur, the display needs to be presented discontinuously – it may not be visible, but the wheel (or anything in a movie or on TV for that matter) moves in jerks. If these jerks occur fast enough (e.g. around 20 times per second) our visual system, namely its motion department, interpolates the intervening missing positions.
This interpolation relies on a “nearest neighbour” principle – thus, if the displacement of the wheel spoke from frame to frame is so large, that it is closer to the (former) next spoke than to its (former) original, our visual system assumes that the opposite motion direction. This implies, of course, that the spokes are fully identical.
Similar effects occur with stroboscopic lighting (see next pages), and indeed this is used to measure the speed of rapidly rotating machinery, for instance an airplane propeller blade. Technically, the entire effect is a case of “aliasing”.