如图１所示，一束平行光由凸透镜折射入一个方框内，要使射出的光仍平行，应在框内如何放置一个透镜，作出光路图．分析这是一道考查同学们灵活运用凸透镜、凹透镜知识的综合题．许多同学仅考虑到框内放置一个凸透镜，使两凸透镜的距离等于两透镜的焦距和（ｓ＝ｆ１＋ｆ２）即 As shown in Fig. 1, a parallel beam of light is refracted by a convex lens into a square. To make the emitted light still parallel, how to place a lens in the frame and make a light path diagram. Analysis This is a comprehensive question that examines the students’ flexibility in using convex and concave lenses. Many students only consider placing a convex lens in the frame so that the distance between the two convex lenses is equal to the focal length of the two lenses and (s=f1+f2).