1．会聚阳光让凸透镜正对着太阳光,拿一张白纸在它的另一侧上下移动,直到纸上的光斑最小最亮为止．此时用刻度尺量出透镜到光斑的距离就近似为焦距． 1. Converge the sun and let the convex lens face the sunlight. Take a white paper and move it up and down on the other side until the spot on the paper is the smallest and brightest. At this point, using the scale to measure the distance from the lens to the spot is approximately the focal length.