A Lightweight and Educational Simulation of Gravitational Lensing by an SIS Galaxy Cluster Using Python

Strong gravitational lensing probes both cosmology and astrophysical structure, yet students and newcomers often encounter steep technical barriers. Here I present a carefully constructed, educational Python simulation of strong lensing by a Singular Isothermal Sphere, tailored for maximum clarity and pedagogical value. The simulation visualizes both the geometric configuration (Einstein radius, source position) and the resulting lensed image of a Gaussian background source. It reveals key features of strong lensing—multiple images, arcs, and Einstein rings, using pure analytic relations, robust logic, and modern visualization. The code relies only on numpy and matplotlib , making it exceptionally accessible for classroom or outreach use. The approach provides an ideal starting point for teaching, rapid demonstration, or as a foundation for further lensing model exploration in research or public science settings. All code and the demonstration figure are published with this article (see Code and Data Availability).