The picture shows a multi-purpose flashlight made by Xiaoyang. AB is a small socket for inserting film, CD and EF are telescopic sleeves, and DF is filled with a convex lens.

Focus, right, upside down, take off the convex lens on the cover and use it as a magnifying glass;

Test and analysis: the light emitted by the bulb is refracted by the convex lens into parallel light, and the parallel light can converge to a point after being refracted by the convex lens. The solution can be considered by using the reversibility of the light path. When the light becomes parallel, the distance (focal length) between the bulb and the lens is now used as a slide projector. If the object distance (the distance between AB and DF) is greater than the focal length, the moving direction of the sleeve can be determined. According to the law of convex lens imaging, real images are inverted. At this time, it is only the case of convex lens imaging. According to different object distances, different applications can determine other functions of the flashlight. The light parallel to the main optical axis is refracted by the convex lens and then converges to one point, that is, the focus. Therefore, according to the reversibility of the light path, if the light emitted by the filament is refracted by the convex lens, it is necessary to put the filament of the small bulb at the focus of the convex lens.

When used as a flashlight, the distance between the bulb and the convex lens is equal to the focal length. Now it is used as a slide projector. At this time, the distance between the slide AB and the convex lens should be more than twice the focal length, while the distance between the bulb and the convex lens is equal to twice the focal length. Therefore, the distance between AB and convex lens should be increased, that is, the sleeve should move to the right.

The slide projector looks like an enlarged real image of an inverted head.

Because the sleeve is a convex lens, the convex lens on the sleeve can be detached and used as a magnifying glass.

Comments: In this problem, the distance (focal length) between the bulb and the convex lens was determined when the bulb was made, but now the distance between AB and the convex lens is equal to twice the focal length. Clarifying the relationship between them is the key to determine the direction of sleeve movement.