Three ray tracing rules for converging lenses 1. Rays coming in parallel to axis converge at the second focal point. The ray that passes through the center continues on straight. The ray actually gets a kink in its motion. To figure out how to grind the lens, one should know Snell's law and mathematics to determine the curve.
Parallel rays rule An object at infinity is imaged on the focal plane.
Large, long focal length, converging lens Smaller source. Do ray tacing. Object closer than focal point, image is virtual, larger, not inverted. Three ray tracing rules for diverging lenses 1. Parallel rays coming in parallel to axis appear to come from the first focal point.