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Xiao Wu drew a u-v diagram according to the object distance and image distance. At this point, the focal length of the convex lens is
(1) When the object distance is equal to twice the distance, it becomes an inverted real image, and the object distance is equal to the image distance, which is equal to twice the focal length. According to the analysis diagram, when the object distance is equal to 20cm, the image distance is equal to 20cm, so the double focal length is equal to 20cm and the focal length is 10cm. ..

(2) When the object moves from 30cm away from the convex lens to 15cm, the object distance of the convex lens is greater than the focal length and becomes a real image. When the convex lens becomes a real image, the object distance becomes smaller, the image distance becomes larger and the image becomes larger.

(3) When the object distance is 8cm from the convex lens, the object distance is less than twice the focal length, forming a vertical magnified virtual image, which cannot be accepted by the light screen.

So the answer is: (1)10; (2) getting bigger; Become bigger; (3) no.