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stroboscopic eyes



> I do not believe that a naked eye without a device of some kind can produce 
> the stroboscopic effect. Ludwik.  

My own restricted  definition of the stroboscopic effect is "an illusion of 
seeing an object in some kind of continous periodic motion (tuning fork, 
rotating wheel, propeller, etc.) at a speed which is less than real speed". 
Some light chopping device is always involved. The choping frequency should 
be high (at least 10 Hz) to match the ability of eyes to retain images. 
I was not aware that fluctuations of intensity at 120 Hz can exist in light 
produced by incondescent sources at very high temperatures. 

Blinking to see a swinging pendulum in the same position each time is
not what I would call a stroboscopic effect. CAN SOMEBODY DEMONSTRATE 
THE STROBOSCOPIC EFFECT, AS DEFINED ABOVE, UNDER THE ORDINARY DAY-LIGHT 
ILLUMINATION?

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> ...  the discussion went to seeing wagon wheels in movies seeming to
> roll backwards. Students maintain that they have seen this in cars with
> naked eye. I don't inderstand the mechanism of how that would happen, yet 
> I in retrospect, I think I have seen it, too. Any ideas?

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> I believe this effect is due to vibration.  It is easily seen when looking 
> at truck wheels in the mirror of the car while driving. ... The vibration 
> must cause the eyes to refocus regularly.  The focused spot is refreshed  
> regularly and it strobes.  The constant focusing also requires a 'refresh' 
> but that is ignored by the brain.
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> When you are bored in a large lecture hall (or church or whatever) that has 
> rotating ceiling fans. Try to count the blades. Usually they are moving 
> slightly too fast to do this easily.

> Blink your eyes.  When your eyes close, they retain a still image of the 
> last thing seen for a short time. Concentrate on the image seen in your 
> memory and you can now (maybe) count the blades on the fan when you couln't 
> before. Sometimes it helps to blink your eyes as rapidly as you can,
> repeatedly, and you can actually see the strobe effect. You can try this on 
> a number of moving objects, and retain a still image of a critical event.  
> But, the object cannot be moving too fast.
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