Bend light, on purpose.

Drop a source, a lens, a prism, a mirror, a beam splitter, a screen — anywhere — and watch light propagate by the real laws, not paraxial shortcuts. Every surface refracts by the vector form of Snell’s law, reflects by Fresnel-weighted intensity, and goes total-internal past the critical angle. White light disperses through glass by a true n(λ). Build a telescope, split a beam, trap a ray inside a prism — the geometry is analytic and the physics is honest.