Question: There’s something puzzling about what I see when I look at a rotating automobile tire. It seems intuitive that if the opaque metal “spokes” present more material than the gap between them (like with a car as opposed to a bike), then the faster it rotates the more light from behind the wheel would be blocked. But instead, even with a nearly fully closed spoke design, I can still “see through” the wheel and have a clear view of what lies behind it, i.e., the brake assembly. What gives? KS — Cascabel, AZ
Answer: Three words … persistence of vision. The 60 fps animation above shows a “wheel” with 92% opaque “spoke” and 8% open gap. Watch what happens as the rotation speed gradually increase from zero to 900°/s over a time of 30 s. There will be several points during the accelerating spin where you’ll be able to “see through” to the background image. FYI, that background image shows Yellowknife Bay on Mars, as imaged by NASA’s Curiosity Mars rover.
It’s important to note that exactly when the “see through” happens for your eyes depends on the refresh rate of the display used for viewing the animation, and the inherent delay between the light from the animation hitting your retina and your brain processing the information into a perceived image. That delay can range from 0.01–0.02 s for the average human (no disparagement intended), and is what causes persistence of vision.
And that’s why, even when looking at a real spinning wheel under continuous daylight conditions (with no display refresh rate), you can still “see through” the moving spokes. The eye-brain system has evolved to notice information and ignore noise. So when the gap in the spokes advances you are still seeing where it had been a fraction of a second earlier. The eye-brain system stitches these individual images together into a smooth composite whole, same as what you experience when watching a movie made from still images displaying at 24 fps. The spokes are noise, and the background is information, so that’s what the eye-brain system will focus on.
There’s still some debate about just why this happens. You can read a fascinating discussion of this illusion on the Physics Stack Exchange at:
https://physics.stackexchange.com/questions/201504/why-does-a-moving-fan-seem-transparent
To further illustrate my point, here’s a screenshot triggered about halfway through the animation:
It shows that what’s on your display in this frame is an opaque “spoke” with only 8% of the background image showing — same as every frame in that animation. Anything else you see in the animation (including the brief moments of “see through”) are being created by your eye-brain system. The transparency you perceive is NOT actually present on the display.
The background you see through the wheel will be less vivid than that outside the wheel. That’s because the eye integrates the light it receives over a brief period of time. With “spokes” occasionally in the way your eye receives less total light, so the background looks dimmer.
This illusion is not limited to wheels and fans. You can see the same effect with the propeller of an airplane. Notice how the clouds at the bottom remain visible through the spinning propeller. Video courtesy of Silverchemist from Wikimedia Commons:
KS also asked about how, when riding bikes at night under 60 Hz street lights, the spokes seem to stand still at some speeds and move backward at other speeds. You can see some of that happening in my animation. He also asked about pulse width modulation (PWM), which is used to dim LED flashlights, and also creates weird illusions with moving objects. Both are well known examples of the strobe effect. Those questions will be answered in a future post.
What you think you see in the real world is created by the eye-brain system. That system evolved over millions of years to help us make sense of the outside world. It does a pretty good job in general, but the inside world we perceive can sometimes be illusory.
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