What causes this curious light phenomenon? (video inside)

[Nantes]

Here is the video:

What is the physical explanation to why I see the light make patterns strikingly similar to magnetic lines on that spinning electric fan, depending on how I move the point of view? (the phenomenon happens equally to the naked eye and to the camera).

 

[gneill]

Can you reproduce the effect using a purely incandescent light source?

Many light sources such as fluorescent and LED have inherent "flicker" due to how their power supplies are constructed. The frequency of the flicker acts as a strobe effect for rotating objects, and you get what is essentially a "rolling shutter" effect.

 

[Drakkith]

It's just where the spokes of the front and back guards overlap from your point of view. Pause the video and you'll see it.

 

[A.T.]

What is the physical explanation to why I see the light make patterns strikingly similar to magnetic lines on that spinning electric fan, depending on how I move the point of view?

See:
https://en.wikipedia.org/wiki/Moiré_pattern

 

[Swamp Thing]

It's just where the spokes of the front and back guards overlap from your point of view. Pause the video and you'll see it.

To prove Drakkith's explanation, we can try removing the blades. Then we will be able to see all parts of the front and back guards. If we now set the fan to oscillate (or if we move our head from side to side), the same effect should happen.

If confirmed, this is an example of a Moire effect.

 

[davenn]

See:
https://en.wikipedia.org/wiki/Moiré_pattern

Yes, exactly ! this can be reproduced easily with sets of crossing lines

Looking out one of my home windows through a fine flyscreen mesh, and looking beyond to a
outdoor aircon unit on my neighbour's house, I see similar patterns as my move my
head sideways as the lines on the flyscreen and on the aircon grill interact

 

[sophiecentaur]

If confirmed, this is an example of a Moire effect.

Presumably the OP first saw this effect and then tried to video it. Does he say he saw the effect with the fan turned off? Did it happen with natural light (from a window)?
I am sure those two factors are distractions. @A.T. 's quoted image demonstrates the pattern without introducing any form of flicker. The OP should do some experimenting with fabrics, combs etc. to produce Moiré patterns in other situations.

But his point about 'Magnetic lines of flux' should also be addressed. Any volunteers?

 

[Frodo]

You see the same Moiré fringe effect when driving and you approach a bridge with handrails held by vertical rods (say, 1 inch in diameter, 6 inches apart) crossing the road.

The near side rods and the far side rods "bunch together" and you see horizontally moving bands of brightness and darkness.

 

[Swamp Thing]

Does he say he saw the effect with the fan turned off?

The fan needs to be running, I believe. Otherwise, the blades will hide the back guard. With the blades running, the back guard flashes onto the retina often enough that its image is able to (spatially) interfere with the front guard's image.

 

[sophiecentaur]

You see the same Moiré fringe effect when driving and you approach a bridge with handrails held by vertical rods crossing the road.

It's interest that the perceived rate of movement is not what you'd expect at that distance - a sort of false parallax effect - and it grabs the attention by peripheral vision. (Your instinct tells you it could be a large predator.)

 

[Frodo]

It is almost certainly because the fan casing is rotating about a vertical axis causing the wires on the front to interact differently with the wires at the back depending on the angle.

The back wires sometimes align side-by-side with the front wires, blocking 'two wire widths of light' and causing a "dark" area. Sometimes they align perfectly one in front of the other, blocking only 'one wire widths of light' so the area appears "light". As the back wires are further from the eye than the front wires they subtend different angles to the eye causing the effect. The curved shape of the wires gives rise to the curved shape of the Moire fringes.

Stopping the fan casing from rotating will prove the matter - it should stop ... unless you move your head.

Remove the fan blade completely but allow the fan casing to rotate and it will still happen.

There is a second potential effect of stroboscopic interaction between the fan blades and the electric light source similar to why wheels on vehicles seem stop or rotate backwards on TV and films.

A third effect is the curved fan blades appearing to merge with the wires when the wires become "close to parallel" with the fan blade edges.

 

[mpresic3]

When I was a young teenager in 1968-9, I remember seeing this pattern, and it reminded me of op-art, like in the opening of the Green Hornet, TV show. I also remember this pattern was seen in a box of stationery paper. The box a a cover with a "target" of perhaps 50-60 concentric dark circles. What was slipped over the box was a plastic cover, open at the two ends that was slipped over the box. The plastic cover that went over the side of the box with the concentric circles was a transparent overlay of another "target" of 50 to 60 concentric dark circles. The effect of sliding the plastic cover over the box to close the stationery would show the moving pattern of "bee" bodies, that you see in the fan. In the common parlance of young teens in 1968-9, as I was a cohort of this age group, was: This was really groovy. Hope this helps