n1 sin θ1 = n2 sin θ2
A drawing machine for light. Every ray on this bench is traced through real glass by Snell's law — drag a lens and the image obeys, tip a prism past the critical angle and the light folds instead of leaving. Nothing is keyframed. The physics is the animation.
Drag glass along the rail. Scroll — or press ↑↓ — on a piece to shape it: focal length, slit, or angle. Drag a piece well above the rail to shelve it. Tab reaches every piece; ←→ slide it.
The bench needs a canvas to trace its rays — your browser declined. The plates below describe each instrument the bench recreates.
Darken the room and prick a hole. Every point of the world sends one ray through, and the far wall receives them upside-down. No glass, nothing to focus — only geometry, and the price is paid in light. Aristotle noticed it during an eclipse; Vermeer, perhaps, built rooms around it.
William Hyde Wollaston, 1807: a prism on a brass stem that folds the scene down onto your page, so the pencil can chase a ghost. Artists bought thousands and complained bitterly — the ghost slides the moment your eye drifts. Total internal reflection does the folding; no silvering required.
Two convex lenses, nothing else. The objective gathers a distant bundle and knots it at its focus; the eyepiece unknots it into parallel rays again — arriving steeper than they left. That steepening is magnification. Kepler described it in 1611 without building one; others obliged within the year.
Newton, 1666: the prism bends violet harder than red, and one ray leaves as a fan. On this bench the dispersion is exaggerated threefold — real glass is subtler than a demonstration wants. Switch the lamp to white and the bench shows its colours everywhere, even where you'd rather it didn't.
Ibn Sahl, Baghdad, 984 — the sine law appears in On Burning Mirrors and Lenses six centuries before Snell measured it and Descartes published it. Credit, like light, bends toward the denser medium.
The lenses here are true circular arcs, so the bench inherits their vice: marginal rays focus short of paraxial ones. Set the iris in front of a lens and close it — watch the image tighten as the outer zones go dark. That is stopping down to ƒ/8, in one sentence.
Set Plate II and let the folded light rest on the paper a moment. Drawing machines dislike being watched, but they do like being waited for.