Reflecting an interval repeatedly across its endpoints maps a reflected path to a free path in an image interval. A return trajectory from ends at after an even number of wall reflections, or after an odd number. An infinite square well has reflection amplitude , so the corresponding Dirichlet heat kernel on an interval is the difference of direct and mirrored free kernels. Individual straight paths illustrate the geometry; the path integral sums all time-sliced paths with the same endpoints.
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The Method of Images is a mathematical technique used in electrostatics, fluid dynamics, and other areas of physics to simplify the problem of finding potential fields due to charges or other sources in the presence of boundaries. It leverages the principle of superposition and symmetry to replace complex boundary conditions with simpler ones by introducing fictitious charges (or "image charges") in calculated positions.