The wheels of a fast moving car seems to be rotating in reverse direction. Scientific term for this effect?

Question

One can see this happening in a movie also when a car is moving with very high speed its wheel seems to be rotating in reverse direction.

Answer 1

Frequency Aliasing

Answer 2

Stroboscopic effect would be a semi-scientific term.
This is caused by the fact that most films have a rate of
about 24 frames per second. When the Wheels of a car
are rotating very close to a mulitple of 24 revolutions per second
but just a teensy bit faster or slower (depending on the direction
of the car and of the film) it can appear to be moving slowly
forward or faster.

You can see very similiar effects with a strobe light, or in
a very simplistic way (no backwards/forwards) with a telvision
which refreshes at 60 frames per second (with USA tv called
the NTSC standard). Move your spread out fingers very rapidly
in front of a tv screen showing something, and in a dark room
it will look like they are jumping from one spot to the next.

sdv

Answer 3

The two above me are wrong. The answer you are looking for is aliasing. This is the same reason that wheels appear to spin backwards, and cd players sample at 44 kHz.

Answer 4

It si because of the FPS (Frames per second) that the human eye can keep up with, the limit for us is around 50 FPS - your tv will usually work at around 24-25 fps.

Some animals such as flies can see upto 200 fps, to them your tv is just a series of still images, whereas to us it just looks like it is moving.

Answer 5

It's called 'frequency folding' or 'sample aliasing' and it happens in sampled data systems when the Nyquist sampling rate criteria is not met.

Doug

Answer 6

I think its called "persistence of vision". The same principle which is used in movies and flip books.

Answer 7

optical illusion