Light and matter have two faces — sometimes acting like a wave, sometimes like a particle — and no single experiment shows both at once.

Light and matter are both wave and particle, but never both simultaneously in a single experiment.

Think of a coin that can land heads or tails, but never both in the same toss. An electron is similar: in one experiment it spreads out like a wave, in another it acts like a tiny ball. Bohr called this complementarity: both faces are real, but you see them one at a time.

Why it matters The idea changed how scientists understand what it even means to "measure" something.