The way to solve the problem: (1) The light parallel to the main optical axis is refracted by the convex lens and converged into a focal point, and the light continues to propagate to the focal point, forming a light spot as big as the convex lens. According to the mathematical congruent triangles, the distance from the convex lens to the focal point is equal to the distance from the facula to the focal point, and the distance from the focal point to the optical center is the focal length of the convex lens.

(2)① According to three conditions of convex lens imaging: U > 2f, inversion and reduction of real image.

2f > u > f, in the inverted and enlarged real image.

U < f, an upright magnified virtual image.

② Short-sighted glasses are concave lenses, which have divergent effect on light, so that the light originally converged and imaged is misunderstood.

(1) The light rays parallel to the main optical axis converge on the main optical axis after passing through the convex lens, which is the focus of the convex lens. When the light passes through the focal point, it continues to spread outward, and a spot as big as a convex lens is obtained. The distance from the convex lens to the focus is equal to the distance from the spot to the focus, so the focal length of the convex lens is [l/2].

(2) Place the candle at point A in the figure, where U > 2f, to form an inverted and reduced real image.

Short-sighted glasses are concave lenses, which have divergent effects on light. Short-sighted glasses are placed in front of the convex lens, which misleads the originally converged imaging light. Therefore, a clear image should appear on the screen, and the screen should be far away from the convex lens.

So the answer is: (1) [l/2]; (2) atrophy; Stay away; Concave lenses have a divergent effect on light.

Comments:

The examination center of this topic: convex lens imaging law and its exploration experiment.

Comments on test sites: correctly understand the converging effect of convex lens on light and the diverging effect of concave lens on light;

The converging function of the convex lens is to narrow the original optical path, so that the converging rays converge in advance, the parallel rays converge into points, and the divergent rays become parallel rays or less divergent rays.

The divergent function of concave lens is to widen the original optical path, and then converge the converged light by mistake, so that the parallel light becomes divergent and the divergent light becomes more scattered.