The Opaque Projector: The Inverse of the Camera Obscura

Many years ago I was running the standard laboratory experiment on thin lens optics. The source was the usual self‐illuminated object mounted on an optical bench, and a converging lens formed a real image on a screen. One of the students sitting near one wall of the darkened lab was having some trouble with the idea of image formation. Her face was lit by the light from a shaded gooseneck lamp, and as she looked at me holding the lens in my hand, the inverted image of her face appeared on a nearby wall. And, the image was in color! Not only was this a classic teaching moment, but I realized that, by chance, we had set up an opaque projector.