Why sometimes see the car on the road ahead, and the wheel looks in the reverse?
If you look at the wheel through the movie, because the movie is 1 second 24 frames, if the wheel happens to be 24 times per second or multiple, then you look at the small point on the static. If the wheel turn a little faster, the small point to move forward (and the wheel direction of rotation), the wheel turn a little bit later moved.
Generally should appear in the evening, because the street is a flash, assuming that the wheel above the uniform 9 spokes, two spoke angle of 40 degrees, the mains frequency is stable 50hz, then the wheel in 0.02 seconds turn n * 40 degrees, (N is a natural number of the line) If N = 1, then it is 0.18 seconds a lap, economic car wheel circumference count 2 meters, then, as long as the second speed of 10 meters (36 km per hour) can be in the street lights under the phenomenon of static, and then a little bit slow to see the "reverse" phenomenon. The less the spokes, the higher the threshold of the "reverse" speed. If it is 24 frames per second camera, the demand is low, probably as long as 18 km or less can begin to observe the reversal phenomenon. I think the answer to the students can understand it ... ...
This phenomenon is called Aliasing in signal analysis processing. According to Nyquist sampling law, when the sampling frequency is more than 2 times the signal bandwidth, the signal can be completely recovered from the sampling sample. Wheel rotation can be seen as a signal source, with (speed * spoke number / 2 / pi) frequency output signal f, human eye sampling frequency fs, sampling results sent to the brain after the original signal is restored. When fs <2 * f, the data in the signal f can not be all collected, reflected in the frequency domain is F in the expansion along the Fs overlap. In other words, all components greater than the fs / 2 frequency are folded into fs / 2 or less, so the result of the sample appears in the original signal does not exist in the signal component, it is called Aliasing.
