人教版初中物理第一册第99页有这样一道习题:“有一个不知焦距的凸透镜,你用什么办法可以粗略测出它的焦距?”这里介绍几种方法。a.会聚阳光法。让凸透镜正对着太阳光,拿一张白纸在它的另一侧来回移动,直到纸上的光斑最小最亮为止。用刻度尺量出透镜到光斑的距离就近似为焦距。 There is an exercise in the first edition of the PEP Junior Physics Book on page 99: “Is there a convex lens that you do not know about the focal length. How can you roughly measure its focal length?” Here are a few examples. a. Convergence of sunlight. With the convex lens facing the sun, move a white sheet of paper back and forth on the other side until the spot on the paper is the smallest and brightest. Measuring the distance from the lens to the spot with a ruler is approximately the focal length.