MadSci Network: Physics

Subject: Interference pattern with a fan?

Date: Sat Sep 9 22:32:04 2006
Posted by Sabry
Grade level: undergrad School: Faculty of Sciences - Cairo university
City: Cairo State/Province: No state entered. Country: Egypt
Area of science: Physics
ID: 1157866324.Ph


I was watching my ventilator (the standing type, fixed grill but the fan 
itself keeps rotating). As it was just in front of a white wall, I could 
clearly notice on the FAN ITSELF (I mean the "fan's screen") small bright - 
dark alternative fringes, so I couldn't help thinking about interference. The 
fringes were circular around the center of the stationary grill, and as the 
fan rotated away from me, the fringes themselves kept increasing in number, as 
if they were coming from the center. As the fan rotated towards me however, 
the fringes kept going into the center. Once I was directly in front of the 
fan, there was just 1 or 2 bright fringes around the center of the grill. The 
lighting was a regular lamp from the ceiling I think.

Also, when I stood up right in front of the fan, I could notice that, as the 
fan was in the east and rotated towards me, as I said, the fringes were 
being "swallowed" by the center, but just after it began to pass me, going to 
the west, the few remaining fringes around the center first rotated about 360 
degree I beleive, then increased in number, from the center.

I'm sure this is a very normal phenomenon, so I hope my description isn't 
bad... if so, you can open a fan from this type and you will immediately 
notice what I'm talking about. So, is it interference? But the fringes are on 
the fan's screen, not the wall... even if so, where are the "slits"? Are they 
formed as the arms move very fastly relative to the stationary grid, thus 
allowing small "slits"?

Thanks very much

Re: Interference pattern with a fan?

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