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Appearance of going backwards?


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Have you ever noticed that when an airplane propeller is accelerating, at some point the blades appear to begin spinning backwards from actual rotational direction. It then smooths out to a stop and then appears to be at full speed. The same effect can be seen on cars with fancy wheels.

 

What exactly is going on here? :confused:

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our eyes can only "refresh" about 30 times a second (as an analogy, TV screens refresh 62 times a second)

if, within that thirtieth of a second, the propeller makes almost one complete revolution (or just less than any multiple of one over however many sides there are on the propeller), then your eyes will see it as if it moved in the opposite direction a bit.

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our eyes can only "refresh" about 30 times a second (as an analogy' date=' TV screens refresh 62 times a second)

if, within that thirtieth of a second, the propeller makes almost one complete revolution (or just less than any multiple of one over however many sides there are on the propeller), then your eyes will see it as if it moved in the opposite direction a bit.[/quote']

 

You need to have a source of light that is blinking on and off, like fluorescent lights. It's a strobe effect.

 

There is such a thing as persistence of vision, but to say our eyes "refresh" is overly simplistic.

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posts #3 and #4 both have truth in them :)

indeed a strobe makes this effect more marked, and persistance of vision or image retention are better words.

 

but the part about Phase is also correct, as a slight increase or decrease in rpm the effect will be forwards or backwards buy the same speed increment/decrement from perfect phase with our image retention capacity, of course at greater speeds this becomes less evident untill we hardle see the object at all just a slight dimming of the light that passes through it (thinking propeler blades here).

we "snap shot" one image and then another, and in phase the blade is in the same place, a picture builds and we think we can see the blades, in actualy fact, depending on the harmonics, we would only see a mixture of all blades, if there were 4 and one had a letter on it, you`de only see it faintly (a quarter).

 

as for a more conscise answer, I lack the words, other than phase harmonics at play with our image rettention ability.

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  • 3 weeks later...

To see the effect, you need to either see it on TV or film (anything with a fixed refresh rate) or under artificial lighting, particularly flourescent lighting (which also has a flicker of around 60 hz).

 

As swansont said, it's a strobe effect. You can't see it in natural light. Whilst retinal cells do have a kind of refresh rate, they don't all refresh in phase. Moreover, the 'refresh' is buffered by the action of retinal ganglion cells. Many photoreceptors project to a single ganglion cell via bipolar cells, and are also connected to each other by horizontal cells (both rods and cones). Bipolar cells are also connected to each other by amacrine cells, so the signal the ganglion cells receive is the net activity of a number of photoreceptors, so it doesn't matter that each photoreceptor is refreshing individually. The net activity remains unaffected.

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how might this be explained then:

 

You see it fairly often if a car overtakes you. I ride buses so this is depressingly often.

as I also have experienced the same thing, and in broad daylight with no internal lighting on the bus or in the car?

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you can see this easily on helicopters, cars, pretty much anything with a propeller or something with a distinguished shape that rotates quickly. it does not require artificial lighting.

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how might this be explained then:

 

as I also have experienced the same thing' date=' and in broad daylight with no internal lighting on the bus or in the car?[/quote']

I have no idea. Temporal lobe epilepsy perhaps?

 

I have never seen this effect directly and outside under natural light. I have seen it on TV, and on film and I have seen the effect in the spokes of the bloke riding next to me at night, under street lighting, and in the propellers of a hercules firing up in a hangar under flourescent lighting for example, but couldn't see the effect on the same 'plane when we landed in daylight. I have seen the effect in the rotors of a helicopter on film and TV, but not whilst next to one, or looking up at them whilst flying in one in daylight.

 

Directly and in daylight, all these things; spokes, propellers and rotor blades, just looked like a blur. Perhaps it's just me.

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I thought our eyes' refresh rate was closer to 120.

 

One can certainly tell the diff between 30 and 60 fps on the computer.

 

...Then again, incandescent and fluorescent lights work at 50-60hz right? In North America at least.

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