This is done so that the slide's image is right-side up.
Work Step by Step
The ray diagram for an image made by a converging lens is shown in Figure 28.47c. The flower is the illuminated slide, i.e., the object to be projected onto the screen. The real image, projected on a screen, is inverted with respect to the object. Therefore, in an old-fashioned slide projector, you inserted your slides upside down.