(2006? Hohhot) as shown in the figure is "to investigate the law of convex lens imaging" experiment, the focal length of the convex lens used for 10cm, the position of the convex lens in the experiment fixed!
According to "when the light screen at 1.5 times the focal length, the screen has a picture", by the convex lens imaging law can be known, at this time, the object distance: U>2f, the image distance 2f>V>f,
And because of the use of convex lenses with a focal length of 10cm, so when the position of the light screen away from the direction of the lens, the screen can be like When the position of the light screen to move closer to the direction of the lens 5cm, the position of the light screen in the focal point, so no matter how to move the candle screen have no image.
The answer is: 5.