(1) If the image cannot be imaged on the screen, there are many reasons, so the answer is:
① The centers of the candle flame, convex lens, and light screen are not at the same place On a straight line;
②The candle is placed at the focus; ③The candle is placed within the focus;
④The candle is placed between one focal length and twice the focal length, but too far away from the focus Near, the image is far away. The light bench is not long enough and the light screen cannot reach the image.
(2) In order to reduce the experimental error and find the exact position of the image, the image must be the clearest, so the answer is: clear.
(3) When a real image of equal size and inversion is presented, the object distance is equal to twice the focal length. As shown in the figure, the object distance is 20cm, so the answer is: 10cm.
(4) As shown in the figure, when the convex lens moves toward the candle, the object distance is within the focus, and an upright magnified virtual image will be formed on the same side of the convex lens.
The magnifying glass is exactly This principle is used, so the answer is: one side of the light screen, magnification, magnifying glass.