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 tunnels 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.