The Zen Photon Garden has a single point light source and partially transparent flat mirrors, everything in 2D.
Other things to add: different refractive indices (to make lenses), shapes curved like conic sections, parallel rays as a light source (though parallel rays could easily be constructed from a point source and parabola). We need a more sophisticated UI.
Since the shapes are simple, can the "garden" be computed directly instead of ray tracing? Multiple reflections probably make things difficult.
Much more difficult: splitting and combining colors of light, diffraction.
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