This Demonstration shows how a planoconvex lens acts on a bundle of parallel light rays. Ideally, a collecting lens would deflect all these rays to meet at a single point. If the rays are parallel to the optical axis, this can indeed be achieved by a convex lens surface in the form of a rotating hyperbola, which was already known to Descartes, Huygens, and Newton. For parallel bundles that form a small angle with the optical axis, the concentration to a point holds only approximately. When deflected into an extended light spot, the oblique rays form surprisingly complex and beautiful patterns that can be studied by mode set to "point image". Compared with the usual spot diagrams delivered by optical design software, the dots are connected by lines. This allows us to trace the dotproducing ray from its starting point. Since the intersections of the rays with the last lens surface are arranged to form a spiral (setting mode to "spiral" shows this), the dots in the image plane form an image of this spiral that can exhibit surprising features. For large openings (small numbers), some rays will undergo total reflection and thus not reach the image plane. In this case one finds the image curve interrupted. Setting mode to "lens" shows a total view, producing the lens, some rays, and the imaging plane. Note that the light comes from below, for example, in a conventional microscope. The action of each control is described by a tooltip.
This Demonstration shows that the ideal onaxis focusing property fades away farther from the axis. So, to show optical aberrations in a simple optical system, a planoconvex lens with a spherical surface can suffice. With the hyperbolic lens, however, much of the interesting associated history is covered in the brilliant book by Burnett [1]. Aristotle already posed the problem of a perfectly focusing curve, "the anaclastic", for which a hyperbola is a solution. Kepler was the first to find this solution, although without a correct proof. Descartes was the first to give a valid proof that a planoconvex hyperbolic lens (with geometric data matching the refractive index in a manner derived by him) has the property of focusing axisparallel rays exactly at one point. Descartes held the opinion that a telescope with such a hyperbolic objective lens would show "whether there are animals on the moon". He did not foresee the limitations caused by the complicating factors of light dispersion and diffraction. The destructive effect of the former—the colored fringes from chromatic aberration—were known to Descartes. He thought, however, that they originate from imperfections of the manual lens grinding process and hoped to get rid of these by ingenious mechanical grinding machines that he designed. He spent much time and energy finding craftsmen willing and capable of building such machines. He thought that using machines would be the only way to make a hyperbolic surface profile with sufficient accuracy. It may be surprising that most modern texts on optics say little about hyperbolic lenses. Snapshot 1: a hyperbolic lens in which the hyperbola is close to its asymptotes Snapshot 2: on axis, where the computed rays intersect the image plane; the deviation from perfect focusing is due to numerical noise Snapshot 3: only 0.1 degrees offaxis: the computed rays form an extended light spot dominated by a coma Snapshot 4: 0.3 degrees offaxis and 4% out of focus: astigmatism becomes noticeable Snapshot 5: where the computed rays intersect the hyperbolic lens surface [1] D. G. Burnett, Descartes and the Hyperbolic Quest: Lens Making Machines and Their Significance in the Seventeenth Century, Philadelphia: American Philosophical Society, 2005.


