This study formed part of a project aimed at revising the instructional approach for geometrical optics in the 10th grade. The instructional intervention was based on the extensive use of a diagrammatic representation as a descriptive, explanatory, and problem-solving tool in the domain. The purpose of this study was to elicit the conceptions and representations of light propagation, image formation, and sight typical to preinstruction learners, with special attention to identifying precursors of problematic features of postinstruction students' knowledge. The premise for this study was that the difficulties students have before, during, and after traditional instruction with respect to representing optical phenomena have their origins in the fragmented prescientific knowledge constructed on the basis of experience. We believe that the difficulties persist because the key factors leading to fragmentation are not usually addressed and remedied. The main findings of the study indicate that (a) preinstruction students display some familiarity with optical systems, light propagation, and illumination patterns; (b) student-generated graphical representations describing and explaining optical phenomena display some features of formal ray tracing; (c) preinstruction students have not developed a consistent descriptive and explanatory model for light propagation; and (d) the context of sight seems to have a confounding effect on the establishment of a unified prior model for optical phenomena.