This is a high school computer lab activity developed specifically for use with the PhET simulation Geometric Optics. It contains a two-page student lab guide with a ten-point formal assessment at the conclusion. The primary objective is to promote understanding of convergent lens optics using a 3-ray system (parallel-focal, focal-parallel, central). Directions are explicit, allowing the flexibility to conduct the activity in either a school computer lab or at a student's home. SEE RELATED ITEMS on this page for a link to the accompanying simulation. This resource is part of a larger collection of interactive Java simulations for students of physics, developed by the Physics Education Technology project at the University of Colorado.