Straight, bent or tilted?
Moiré pattern
Where do the big dark curves in the middle come from?
Nobody drew them: they appear where the two sets of rings overlap. You see the same thing through two layers of curtain or mosquito net.
Why? Where the lines of the two patterns fall on top of each other, the paper looks light; where they fall in each other’s gaps, it looks dark. Those light and dark areas make new, much bigger patterns.
Picture: SharkD, CC BY-SA 4.0 · Wikipedia(opens in a new tab)