Why is gold yellow?

DAVID MIEDUNIECKI , FRANCE

Simple chemistry predicts that gold and silver should have the same silvery appearance. To explain gold’s colour we need something else – a mix of quantum mechanics and Einstein’s Special Relativity.

Quantum mechanics describes an atom’s electrons sitting in discrete orbitals. In the case of silver, it takes a high-energy, ultraviolet photon to kick an electron up to a higher orbital. Lower-energy, visible photons are reflected back so silver acts like a mirror. Relativity comes into play because, due to the size of gold atoms, its electrons are travelling at over half the speed of light. Einstein’s theory tells us that at these speeds the mass of the electrons increases, which in turn means the energy needed to kick them up to another orbital is reduced. So lower-energy blue photons are absorbed, and don’t get reflected by the gold. And if blue is removed, we see yellow.