Why is it that in natural sun light, when cars tires or helcopter blades speed up and slow down, They appear for a short time to be rotating in reverse? I’m talking real-world here. Not movie or TV. So it cannot be due 60 hertz artificial lighting.
My top theory.
Brain frequency? I have no idea what that would be, could you please expand on this concept?
That’s not what Cecil says:
http://www.straightdope.com/classics/a3_199.html
Personally, I buy it. I’ve even noticed the effect on overcast days when streetlights are on.
Um, have you tried adjusting your front side bus? <g>
Anthros
(overclocked but crash-prone)
I read the cecil post.
And he is completely wrong.
It happens in real life, day time light all the time.
Practically, everytime I see a helicopter take off or land.
I’ve also seen it on cars all the time.
Cecil’s answer is poor.
Surely, others out there have noticed this occurence.
Another good example is with a small fan.
Like one of those hand cooling fans.
About the eye/brain frequency. I have no support
I was trying to think of something.
Care to support this assertion? Like, why do you think he is wrong? I mean, besides “it happens.”
Anthros
I have not noticed it in natural daylight, only in artificial light. How can I check my brain frequency?
Brain frequency is h/(brain wavelength). Simply extract your brain, pound it into a flat tortilla shape and measure the length, do the math, and voila brain frequency.
Hmmm, seems like alot of trouble. Maybe it’s better to be ignorant about some things.
I have also noticed this effect in real life. How many frames per second can the human eye/brain process?
My guess would be infinite since the world is, well continuous. When you ask about frames per second you are trying to put the human brain into a mechanical frame of refernce, and it doesn’t work that way.
This might have something to do with it.
“Although an image on the retina decays gradually, rather than lasting a specific amount of time, there is a critical period during which the stimulus changes so little that the visual system cannot take in any new information even if the eyes are open. This period, on average, is about 50 milliseconds, or one-twentieth of a second. Thus, the average human visual system can only take in about 20 different images per second before they begin to blur together. If these images are sufficiently similar, then the blurring which takes place appears to the eye to resemble motion, in the same way we discern it when an object moves smoothly in the real world.”
http://www.ece.wpi.edu/infoeng/textbook/node71.html
I have seen similar effects in broad daylight. One was when looking at oncoming trafic from a bus window. It turned out that the bus window was covered with plastic film with regularly spaced stripes. A bit like those cylindes with slits, with pictures drawn inside, so when you spin the cylinder and look through the slits you see an animated image. (You must have seen them in science museums, right?)
Also, when you look at propellers at an angle, at some point the blade is edge on and almost invisible, while 90 degrees away from this point the blade is face on and visible. Thus if the propeller was spinning fairly fast, you will see two blurs, as if it were a stationary 2-blade propeller. If the plane was moving, the viewing angle changes, and so does the angle of this fake propeller. It may happen with helicopter rotors and certain designs of car wheels.
A very similar effect can happen with reflected sunlight; that is, at one point (or two) during the rotation the blade reflects sunlight in your direction, so you also see a fake stationary blade at that point. Again, as the plane/helicopter moves by, the angle of this changes.
I can’t think of any other reason this effect can happen in daylight. Even if the eye takes discrete images 20 times a second, the exposure time of each image is 1/20 seconds - that is, the shutter never closes. The stroboscopic effect only happens if the shutter speed is much faster than the frame interval. That is, the duty cycle (the fraction of time that the shutter is open) must be fairly low.
I estimate the brain frequency on this board is about one out of every 3.2 posts.
Bahahahahahahaha!