Head Rotating Lights
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Why is it sometimes vehicle wheels appear to rotate in the opposite direction from travel on the motorway?
Haven't you ever noticed this, how a decelerating high speed rotating part can appear to slow, pause and appear stationary and then appear to reverse direction, and then continue its deceleration?
Is it something to do with the frequency of light and the rpm of the object?
Why when you blink and stare at the rotating object can your eye take a stationary snapshot of the object?
All answers or theorys are appreciated, if only so that i stop standing at the roadside blinking at the traffic and scratching my head. Carl.
It's an effect called Aliasing. It's due to when the wheel reaches what's called the Nyquist frequency.
When the spokes are rotating at a rate just fast enough, one spoke will appear to be just behind the previous one, but overlapped enough to give you the illusion that the spoke has just moved backwards slightly.
This effect is valid for certain integral values of the Nyquist frequency, so at various specific speeds you will see this effect speed up or slow down. I tried to find you something on the web that illustrates this, but all the applets I found were static, and ridiculous. They would have confused you more than clarify.
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