Image formation by two plane mirrors

Abstract

We revisit the problem of image formation of a point object by two plane mirrors meeting at a convex angle θ. Although a complete solution to the problem already exists in the literature, the simple formula is often assumed to be the universal formula that counts the number of images, irrespective of θ or the location of the object. A survey, involving both school students and teachers, shows that the simple formula in conjunction with rounding off to the closest integer(s) is still widely thought of as the correct way to count the number of images formed. In order to address this rampant misconception, we rederive the correct formula for an arbitrary mirror angle θ and arbitrary location of the object in an elementary way. The proof is easily communicable to high school students. We justify the formula using a physical setup and capturing real-life images. Finally, we provide a simulation written in an open source software GeoGebra as an additional visual aid in understanding the problem and its solution.