This thesis investigates the effectiveness of two multimedia programs in delivering an understanding of the light - dependent reaction of photosynthesis. One program, Cells and Energy, was adaptive, whilst the other, Photosynthesis Explorer, was interactive (a practical simulation). To inform the value of these different designs an empirical study was conducted. Ten pairs of participants were allocated to use one or other of the programs. During their use and with the researcher's support,

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... rs of each pair attempted to learn about the light-dependent reaction. Whilst doing so, audio and visual data were captured to provide information as to participants' and researcher's activities related to this learning process. Each participant's understanding was determined by matched pairs tests - as a pre-test and as immediate and delayed post-tests. The programs generated a highly significant difference (p < 0.0005) amongst test results, with increased scores in the post-tests, but there was no Significant difference between the programs on participants' performance. Nevertheless the Photosynthesis Explorer group took about three times as long to deliver this equivalent effect. By employing Laurillard's Discourse Model for evaluating events, which were recorded during the programs' use, this research provided evidence of the importance of feedback as scaffolding and support in delivering knowledge and understanding. The recorded, as well as test, data revealed misconceptions. Their effects on learning were complex as were cognitive conflict episodes arising from them, whose resolution was multifaceted.