Question

why the rotating blade of a fan looks like they are rotating in the opposite direction in deemed light?

Answer 1

Search for aliasing, I think that's the effect your talking about…

Answer 2

Look ... Eyes of human .. can take see pulses at certain time and depend on brain how to translate this :
for example .. if you consider that you have "Watch " and its moves so fast .

at first pulse you see 1
at second pulse you see 12
at third pulse you see 9
at fourth you see 6
then 3

1....12....9....6....3

of course logically you will translate this that watch rotate Anti Clock wise !

The same thing happen in the blades of fan .

Hope I'm right .. .... regards

Answer 3

Yep, that's a nice way to understand this phenomenon. Even tough the waves in the nature are continuous, our perception of them is discrete, so, if the frequency of the wave is big, we can perceive it in a different way. This picture might help:

If you look at it, the circulated values are the moments your brain is perceiving the signal, so now the real wave (continuous line) and the perceived wave (dashed line) have different frequencies and phases. As light is an electromagnetic wave, our vision can tricks us that way, hope it helped!