Glass rod
Refracts at every glass-air boundary and supports total internal reflection. Catalogue materials add traced path-length GDD; constant index keeps legacy behavior.
Open in the canvas →In the real world
The geometry OpticalSetup draws for a glass rod is a plane-parallel slab: two flat, parallel long faces and two flat ends — the classic "glass block" of an introductory optics course. At normal incidence, light passes straight through with no net angular deviation but a real velocity change: phase velocity inside the medium drops to c/n, so light takes longer to cross the same physical distance than it would in vacuum or air — the basis of every optical delay produced by inserting glass into a beam path, from picosecond fiber-stretcher spools to the fraction-of-a-picosecond thickness of a camera sensor's cover glass.
At any nonzero angle of incidence, Snell's law bends the ray at entry and bends it back by the same amount at exit — the two parallel faces cancel the angular deviation exactly — but the beam still emerges shifted sideways from where it would have gone straight through, a lateral displacement that grows with thickness, incidence angle, and index. It's the same "apparent depth" effect that makes a straw look bent in a glass of water, just viewed from the side instead of from above.
In OpticalSetup
The rod is four independent flat refracting boundaries — two long faces and two ends — each obeying the exact vector form of Snell's law and total internal reflection used by every dielectric surface in the app, so tilting the rod at an angle reproduces the real lateral-displacement geometry above, not an idealized straight pass-through. Inside the medium, the tracer accumulates optical path length as geometric distance × refractive index; on the shared pulse-timing overlay this means a packet visibly slows down while crossing the rod, lagging a same-time packet on a vacuum path by exactly the extra delay the formula above predicts for the configured index. The rod's fill is deliberately translucent so that lag is something you can actually watch happen, rather than a number hidden behind an opaque block. Choose the legacy constant index or one of the four catalogue Sellmeier glasses. A catalogue material also accumulates GDD from the actual distance each ray travels inside the rod.
The default remains a single constant index so every existing saved rod keeps its authored behavior; that mode has no material GDD. Selecting a catalogue glass enables Sellmeier refraction and path-length GDD, generally within a few percent where the curve is valid, but still omits absorption, temperature, coatings, and higher-order pulse effects. There's no cylindrical or lensing geometry either: despite the name, this is a rectangular slab cross-section with flat ends, not a focusing rod lens.