To see or not to see ...

[Southwind17]

So, I'm riding on a tram today looking out of the window observing the cars go by (so much for public transport!), and I notice that as each car accelerates the more visible becomes the brake discs and calipers behind the rotating spokes of the wheels, apparently. At slow speeds the said parts seem almost totally obscured by the spokes (obviously, when stationery some parts are completely obscured), but at high speed it's as though the spokes have become almost transparent, like looking through the blades of a high speed aeroplane propellor. Now, the degree of obscurity, on average, is surely constant regardless of the speed of the rotation of the wheel (or propellor), so how do we account for the visibility seemingly increasing with speed?

 

[learner]

So, I'm riding on a tram today looking out of the window observing the cars go by (so much for public transport!), and I notice that as each car accelerates the more visible becomes the brake discs and calipers behind the rotating spokes of the wheels, apparently. At slow speeds the said parts seem almost totally obscured by the spokes (obviously, when stationery some parts are completely obscured), but at high speed it's as though the spokes have become almost transparent, like looking through the blades of a high speed aeroplane propellor. Now, the degree of obscurity, on average, is surely constant regardless of the speed of the rotation of the wheel (or propellor), so how do we account for the visibility seemingly increasing with speed?

I will join you in your curiosity and add that it seems strange to me that the Discs become more visible rather than less visible. Why is it the spokes do not, all but, obscure the Discs?

 

[Zep]

Your eyes have a natural "flicker" or strobe rate if you will of about 23-25Hz which is the smallest interval of change in vision you can distinguish. Which is why the old 19fps silent movies looked flickery, but the current 25fps movies (or higher) don't. After that, your effect is simply a strobe effect, as described beautifully by people like Harry Edgerton.

http://en.wikipedia.org/wiki/Harold_Eugene_Edgerton

 

[Ririon]

What you see is as much a product of the brain's vision processing as what your eyes actually see.

The spokes are a blur and are disregarded. The parts of the brake disk you see at any point are added together to form a complete image of the brake disk.

If the wheels spin fast enough, this even works with a camera. The spokes are an even blur and as such is not recognizable as a wheel. During the exposure time, all parts of the brake disk has been visible at several times.

Do an image search for "glowing brake disk" for some cool imagery.

That is my humble amateur hypothesis, at least.

 

[Dancing David]

That seems to be the best Ririon, my knowledge is 24 years out of date. The brain will get the sensory data on the pads more consistently.

The flicker rate varies amongst people as well from as low as 16hz to the 99th percentile at 60hz.

 

[Ivor the Engineer]

What you see is as much a product of the brain's vision processing as what your eyes actually see.

The spokes are a blur and are disregarded. The parts of the brake disk you see at any point are added together to form a complete image of the brake disk.

If the wheels spin fast enough, this even works with a camera. The spokes are an even blur and as such is not recognizable as a wheel. During the exposure time, all parts of the brake disk has been visible at several times.

Do an image search for "glowing brake disk" for some cool imagery.

That is my humble amateur hypothesis, at least.

I agree. All our senses have spatial and temporal bandwidth/resolution limits which affect how we perceive sensory stimuli.

E.g., depending on how far away it is from our eyes, a piece of paper with black and white stripes printed on it will either look grey (when it is viewed at a distance) or the individual stripes will be visible (when it is viewed close up).

 

[Southwind17]

Interesting explanations, but none of them seem to address the apparent fact that as rpm increases so does image clarity, ad infinitum, I suspect. I suggest that a wheel spinning at infinite rpm would lead to no loss of image clarity compared to a stationery, spokeless wheel.