在幾何光學課程的安排中，成影概念通常被認為是相當簡單的學習單元。然而，很多研究顯示，多數的學生，包括中學生及文科的大學生，並不能夠從幾何光學的原理來解釋這個現象，他們多數是以影像概念來解釋，也就是認為影子是由於光照射到物體，再從物體發射出來的影像。因此我們有必要針對一般學生學習成影概念的困難原因及成效做深入的探討。 本研究從概念診斷、認知衝突的教學實驗及科學史的探討三方面進行。研究的對象包括中學生、小學生及大學生。主要的結論為： 1. 對於成影概念的瞭解，必須牽涉到很多其它的概念，同時也和學習者本身的能力有關。學生學習的困難在於對一些相關的光的性質不瞭解，及缺乏想像光線變化的能力。2. 如果沒有透過適當的教學，幾何光學的成影概念不會隨著年齡的成長而自然建立，影像發射式的成影概念也不會隨著年齡的成長而消失。3. 用光線作圖方式來決定影子的大小是一般在成影概念的教學上所採用的方法。 但是，對學生來說，所畫的光線並不一定具有光的直線傳播的意義。學生在圖上所 畫的線，通常只是代表“光亮”、“光的照射方向”、“影像發射”、或者只是作 圖時的輔助線。4. 科學史的探討和研究學生概念所得的結果有很多類似的地方，但本質上則有點不同。本研究從概念的特性、定律的發展、實驗的觀察及幾何光學的學習等方面，針 對二者之間的異同做了相當多的探討。基於以上對成影概念特性的瞭解及學習困難原因的探討，本研究所發展的具有認知衝突的教學方法，可以提供教材教法上的參考。

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.