This article describes an investigation of student understanding of geometrical optics and illustrates the use of research as a basis for the development of curriculum. Evidence is presented that university students who have studied physics at the introductory level and beyond often cannot apply basic concepts from geometrical optics to account for the pattern produced on a screen when an aperture or obstacle is placed between a light source and the screen. Identification and analysis of student difficulties guided the initial design of a tutorial to supplement instruction in a typical calculus-based or algebra-based course. Development of a laboratory-based, inquiry-oriented curriculum for precollege teachers took place concurrently. Ongoing assessment was an integral part of the curriculum development process. The instructional materials that evolved from this iterative cycle have proved to be effective with the target populations.