Thin concave lens
Diverges rays with a negative thin-lens focal length. Pulse GDD silently assumes N-BK7 and a diameter-aware sag thickness.
Open in the canvas →In the real world
A concave (diverging) lens obeys the exact same thin-lens equation as a convex one — the only difference is the sign of f. A negative focal length always produces a negative image distance for a real object, which means a concave lens can never form a real image on its own: the rays always appear to diverge from a virtual, upright, reduced image on the same side as the object.
Concave lenses correct myopia (short-sightedness) in eyeglasses, and paired with a convex lens they make a compact Galilean telescope or beam expander — see the telescope page.
In OpticalSetup
This is literally the same component as the convex lens — same paraxial ray-transfer relation u' = u − h/f, same registry entry under the hood — just defaulting to a negative focal length. Setting a positive focal length on this element makes it behave exactly like a convex lens, and vice versa: the sign of f is the only thing that determines converging versus diverging behavior anywhere in OpticalSetup.
Same caveats as the thin convex lens: exact paraxial geometry with no spherical or chromatic aberration. GDD alone uses the same diameter-aware N-BK7 sag estimate (roughly a 10% class estimate); the assumed thickness never becomes traced geometry.