[[abstract]]We begin to teach our children the reason for shadow formation when they are still in elementary school, and few of our elementary or high school teachers consider this a difficult subject to teach. However, many researches have found that many students, high school students and non-science major college students included, fail to understand the theory of shadow formation of geometrical optics. Most of the students use an "emanating" model to explain it. That is, they think that an image is formed near the object when light irradiates it, the image is projected onto the screen and then the shadow is formed. The phenomenon is of course interesting from the viewpoint of learning and teaching theory - what hinders our students from understanding such a common phenomenon and, to us, a simple theory? Three parts are included in this study: the diagnosis of students' conceptions,the learning experiment with the strategy of cognitive conflict, and the study of historical conceptions in optics. The subjects of this research include elementary school, high school, and college students. The major conclusions areas follows:1. Many other concepts are involved in the understanding of shadow conceptions.The learners' cognitive development ability is also related to it. Students' learning difficulties lie in their various misconceptions of light properties and their lack of mental abilities to imagine the variations of the direction of light rays. 2. Without proper teaching strategies, students do not spontaneously acquire the conception of shadow formation of geometrical optics as they grow up. Also the conception of emanating image will not fade away as they grow up, either. 3 . Drawing light rays to obtain the size of the shadow is the traditional way in teaching shadow concept. Nevertheless, the light rays students draw do not necessarily mean light propagation. The lines in their drawing usually simply represent "brightness", "the shining direction of the light source", "the emanating image of an object", or "supplementary lines in drawing".4. Many similarities are found in the research on the study of students' conceptions and the historical development of optics. However, there is a little difference in essence. To discuss further similarities and differences between them, this study focuses on the characteristics of conceptions, the development of principles, the observations on experiments, and the learning of geometrical optics. Based on the understanding of the characteristics of shadow concept and the difficulties that learners encounter, some teaching strategies of cognitiveconflict are developed in this research, and they may be applied to physics instruction and curriculum design.
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