(2) The purpose of cutting off the candle in the experiment is to make the centers of the candle, the convex lens and the light screen approximately at the same height, and to make the image in the center of the light screen.
(3) The focal length of the convex lens is 15cm, the object distance is 12cm, and u < f, which is an upright magnified virtual image. The optical screen can't accept virtual images, so the optical screen can't accept real images at any position.
When the convex lens becomes a virtual image, the object image is on the same side of the convex lens, so the eyes should observe the candle through the convex lens on one side of the screen.
(4) The image that can be received on the screen is a real image, and the convex lens is inverted and enlarged, 2f > u > f, v > 2f, so the distance between the candle and the convex lens is more than 15cm and less than 30cm, and the distance between the screen and the convex lens is more than 30cm, so it is applied to projectors or slide projectors.
(5) As shown in Figure A, if an image can be formed in the center of the screen, it can be moved down transparently according to the light passing through the center without changing the direction, and an image can be formed in the center of the screen.
(6) After getting a clear image in the previous experiment, Xiao Ming took a pair of myopia glasses and put them in front of the convex lens (as shown in Figure B). In order to form a clear image on the screen, he can keep the candle away from the convex lens to the left.
So the answer is:
( 1) 15cm；
(2) The image can be formed in the center of the screen;
(3) The object distance is less than a focal length; Eyes, convex lenses, candles;
(4) less than; Greater than; Slide projector;
(5) downward;
(6) Left.