《物理教师》1990年第10期对称法解题例析》一文中例3、例4解法欠妥,特提出质疑。原例 4:把一个点光源放在焦距为f的凸透镜的焦点上,在透镜的另一侧2倍焦距处放一个垂直于主光轴的光屏,在光屏上看到一个半径为R的光亮的圆。现保持透镜和光屏不动,而在主轴上移动点光源。若要使光屏上的亮圆半径缩小到R/2,则这个点光源应移到什么位置上? 《对》文作者由几何关系得出u_1=4f/3(相应的v_1=4f)后,写道:“由于物与像 “Physics Teachers” in the 1990th period of the symmetry method to solve problem examples“ article in Example 3, Example 4 solution is not right, special challenge. Original Example 4: Place a point light source on the focal point of a convex lens with focal length f, and place a screen perpendicular to the main optical axis at the focal length of 2 times on the other side of the lens. A radius R is seen on the screen. Shiny circle. Now keep the lens and screen fixed, and move the point light source on the spindle. To make the radius of the bright circle on the screen reduced to R/2, what position should the point source be moved to? The author of the pair is derived from the geometric relation u_1=4f/3 (corresponding v_1=4f) , wrote: ”Because of things and images