Effective teaching of physics requires the use of well-designed and diversified instructional tools such as multimedia throughout the teaching and learning process. The main objective of this study was to investigate the effectiveness of Physics Educational Technology (PhET) simulations and YouTube videos to improve the learning of optics in Rwandan secondary schools. The study was framed by the cognitive theory of multimedia learning. A total of 136 senior-4 physics students from six schools were divided among three groups and taught with the usual teaching methods only, the usual teaching methods supported by PhET simulations or YouTube videos, respectively. Student achievement was measured by administering the Geometric Optics Conceptual Understanding Test to each group of students before and after teaching. The groups of students who were taught using PhET simulations and YouTube videos achieved significantly more gains on the posttest compared with the students who experienced neither. PhET simulations and YouTube videos saw average normalised learning gains of 12 and 11%, respectively, while students who experienced solely the usual teaching methods got a gain of only 2%. These results show that the use of PhET simulations without student manipulation (as applied in this study) is equally effective as the use of YouTube videos. The researchers recommend that teachers incorporate these instructional tools as a way of effectively teaching and learning optics.