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如图1所示,凸透镜L成实象时,如果紧贴透镜处有一半被黑纸遮住,仍能形成物体完整的象。因为物体AB上各点发出的光线均有一半进入凸透镜,只是成象的亮度变暗些。但如图2所示,在凸透镜前相距为d的地方,用黑纸遮住透镜的一部份,从图2不难看出,物体上A点发出的光,完全不能进入透镜,无法成象。而物体上A_1点发出的成象光束中,有一部份仍能进入透镜,成实象点A_1,故成象范围仅为A_1B_1,越靠近
As shown in FIG. 1, when the convex lens L is real, if a half of the lens is hidden by the black paper, it can still form a complete image of the object. Because half of the light emitted by each point on the object AB enters the convex lens, only the brightness of the image becomes darker. However, as shown in FIG. 2, at a distance d from the front of the convex lens, a part of the lens is covered with black paper. It can be easily seen from FIG. 2 that the light emitted from the point A on the object cannot enter the lens at all and cannot be imaged. . Some of the imaging beams emitted from the A_1 point on the object can still enter the lens and become the real image point A_1. Therefore, the imaging range is only A_1B_1.