This study explored high school and teacher-training college students' knowledge of light, vision and related topics before and after commonly practised instruction. This knowledge was analysed and interpreted in the light of premises for the construction of alternative knowledge by learners of optics. A hierarchical structure was suggested to represent the collective conceptual knowledge of students in terms of facets and schemes of knowledge. 'Abundance' and 'gain' coefficients permitted quantitative description of the spread and alteration of the facets and schemes. In place of confronting misconceptions individually, schemes provide a basis for the design of more effective methods of instruction to challenge the fundamental patterns of alternative knowledge. Student misconceptions identified in other studies were included for comparison. On the basis of the study, suggestions are made for modifications in curricula to improve optics instruction.