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Virtual tunnels and green glass: The colors of common mirrors

When a pair of common second-surface plane mirrors face each other, repeated mirror-to-mirror reflections form a virtual optical tunnel with some unusual properties. One property readily analyzed in a student experiment is that the color of objects becomes darker and greener the deeper we look into the mirror tunnel. This simple observation is both visually compelling and physically instructive: measuring and modeling a tunnel’s colors requires students to blend colorimetry and spectrophotometry with a knowledge of how complex refractive indices and the Fresnel equations predict reflectance spectra of composite materials.