Maybe optical illusion is the wrong word, optical effect?
Today I’m sitting next to another car at a red light. It’s wheels were sort of like fan blades so that approximately half of what was beyond the wheel was visible (the brakes, etc).
As we drove away from the light and got up to speed I happened to look over again. This time the wheels sort of disappeared And you could see everything beyond, like looking through a fan that is running. And it made me wonder why the spaces were the default effect. Why isn’t the default effect the solid parts that block all vision?
You’re eyes naturally tend to focus on stationary bits, like the brake calipers and what not. The times thus just seem like a blur in the way. If you purposely chose it to watch the rim spokes, which you can, then the you’d see them fine and the brakes would juseem like fussy garbage in the background. However your eyes would tire very quickly by trying to focus on the spokes going around and around and around and around and around…not something I’d advise while driving. As a passenger, I’ve spent a lot of time running this experiment.
The actual effect is partial transparency, similar to looking through partially-smoked glass. You register it more as looking at what’s behind it, because what’s beyond a window is always more interesting than the window itself.
Consider a spot on the brake cylinder. Your eye and brain are summing the light input from that spot over a finite time period (up to a couple hundred milliseconds!): they will add the light from (brake caliper, uncovered) to (brake caliper, covered by wheel) over that period. You’ll always get the signal from the uncovered caliper superimposed over the signal from the wheel alone. Whether this looks like ‘wheel with hazy brake caliper behind it’ or ‘brake caliper with hazy wheel in front of it’ depends on the relative brightness of those two things and the fraction of time the caliper is uncovered vs. covered over that visual summation period.
It’s like when you’re driving at night with your headlights on, and there is a car driving behind you as well. You can see in front of you how things are less well lit where just your own headlights are hitting them, as opposed to where your headlights and the following car’s headlights are overlapping. As a kid I used to feel bad that our car had such wimpy headlights that any old car behind us could outdo
them. I thought that the bright spots at the sides were just from the car behind, not the sum of their lights plus ours.
I’ve noticed this with screen doors. If you sit perfectly still, your vision is clearly blocked in many places. Then move around, and the screen almost disappears. Your brain is able to combine images from other recent moments in time to be able to piece together the entirety of what it thinks reality is like from a few partially blocked images separated by short distances of time. This is basically the same thing, showing just how fast your brain is able to work piecing together images.