Abstract
We describe light-reflection properties of spherically curved mirrors, such as baubles on a Christmas tree. In particular, we study the position of the image which is formed somewhere beyond the surface of a spherical mirror, when an eye observes the image of a pointlike light source. The considered problem, originally posed by Abu Ali Hasan Ibn al-Haitham—alias Alhazen—more than a millennium ago, turned out to have the now well-known analytic solution of a biquadratic equation, being still of great relevance, e.g., for the aberration-free construction of telescopes. We do not attempt to perform an exhaustive survey of the rich historical and engineering literature on the subject, but develop a simple pedagogical approach to the issue, which we believe to be of continuing interest in view of its maltreatment in many high-school textbooks.
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