Image Formation and Light-Beam Boundaries

For image formation, basically only one condition needs to be fulfilled: The rays which originate at “object points” must be limited to a small spatial opening angle ω by passing through an aperture (Sect. 16.1) on their way to the image plane (e.g. a projection screen or a photographic plate). In Fig. 17.1a, the aperture B is a small round opening (pinhole camera!); in Fig. 17.1b, it is a small planar mirror. In both cases, as a suitable object to be imaged, we have chosen an arrow or letter made with small light bulbs.