笔者在“研究透镜成像”的分组实验课上，常遇到学生问及这样的问题：不管是凸透镜还是凹透镜，不管物距多大（包括物体在凸透镜的焦点上），也不管屏在什么位置，总能在屏上看到物体的像，这是为什么？当时由于时间的限制，我只能做出这样的解释：平时大家用公式法算出的、用 In the grouping experiment of “researching lens imaging”, the author often encountered the question of students asking whether they are convex or concave lenses, regardless of the object distance (including the focus of the object on the convex lens) and regardless of the position of the screen. You can always see the image of an object on the screen. Why is this? At the time, due to time constraints, I could only make such an explanation: usually we use the formula method to calculate