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This Demonstration shows how a plano-convex 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 dot-producing 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.
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