COPPER · GOLD

LITOnly two elemental metals are coloured. Their reflectivity drops sharply where light can lift d-band electrons into the conduction band: about 2.1 eV for copper (≈ 590 nm, so yellow and green are partly absorbed and copper looks red-orange) and about 2.3–2.4 eV for gold (a relativistic band calculation gives 2.38 eV, ≈ 520 nm). Silver's edge is near 4 eV, in the ultraviolet, so it looks white. For gold, scalar-relativistic effects lower the interband onset from about 3.5 eV to about 1.85 eV in DFT (NJP 12, 103048, 2010); without relativity gold would look silvery.

MODELTwo plates in the void. Reflectance is modelled as a smooth step at the edge (95% → 40%); the reflected colour is computed from it. −Orange+: copper on the minus side (edge lower, redder), gold on the plus side. Toggle relativity off and gold's edge jumps to 3.5 eV: its glow goes white.

relativity: ONtilt the plates

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