Wave-Particle Duality Explained Simply

Is light a wave or a stream of tiny particles? The answer of quantum physics is astonishing. It is both at once, and the experiment decides which side we see.

What duality means

For a long time researchers argued whether light was wave or particle. Quantum physics resolved the dispute surprisingly. Both descriptions are right.

Which side shows depends on the experiment. Sometimes light behaves like a wave, sometimes like a particle. This dual character is called wave-particle duality.

The wave picture

As early as 1801 an experiment showed wave behavior. Light falling through two slits creates a striped pattern. Such patterns arise only when waves overlap.

Waves can reinforce or cancel each other. That is exactly what explains the bright and dark stripes on the screen.

The particle picture

In 1905 Einstein explained another effect. Light can knock electrons out of a metal, but only in fixed energy packets.

These packets are called photons. Here light behaves clearly like a stream of particles. For this explanation Einstein received the Nobel Prize.

Why the double slit is so strange

Send single particles one after another through the double slit, and the striped pattern still appears. Each particle seems to go through both slits at once.

This is only explainable with superposition. But if you measure which slit it went through, the pattern vanishes at once.

Why it applies to all matter

Duality concerns not only light. Electrons and even large molecules show wave behavior in the double slit.

So duality is a basic property of the quantum world. It shows how differently the smallest scale works, as the quantum mechanics section shows.