AP® Physics 2: Algebra-Based · Unit 15 · Topic 15.1
Wave-Particle Duality
Light and matter each behave like waves or like particles depending on how you look at them. Neither picture alone is complete; both are needed, and measurement itself decides which one shows up.
Shine light through two narrow slits and you get an interference pattern on a screen behind them, bright and dark bands, exactly what overlapping water waves would produce. Fire electrons one at a time through the same setup and the same banded pattern slowly builds up, dot by dot. That is strange, because electrons are supposed to be particles. Each one lands at a single point, like a bullet hitting a target, yet thousands of single landings paint out a wave pattern. The resolution is that a photon or electron does not travel as a tiny ball on one definite path. Between source and screen it is described by a wave function, a spread-out set of possibilities that passes through both slits at once and interferes with itself. Only when it hits the screen does it commit to one spot.
This is wave-particle duality: light and matter each show wave behavior and particle behavior, and which one you see depends on the experiment. Interference and diffraction reveal the wave side. Photon-by-photon detection, with each hit a sharp dot, reveals the particle side. Neither description alone is complete.
Put a detector at one slit to see which path each particle actually took, and the interference pattern vanishes, leaving just two overlapping bright bands, the classical result you'd expect from particles going through one slit or the other. Measuring the path forces the particle out of its superposition of both paths into a definite one, destroying the self-interference. This sets up the next idea, that confined quantum objects like electrons in atoms only sustain standing matter waves at certain discrete energies, the energy ladder of quantized states.
Key terms
- Wave-particle duality
- The fact that light and matter each display wave behavior, like interference, and particle behavior, like discrete detections.
- Wave function
- The mathematical description of a quantum object's spread of possibilities before it is measured.
- Superposition
- A quantum state combining multiple possibilities, such as both slit paths, at once until measurement.
- Interference pattern
- The alternating bright and dark bands produced when waves from two slits overlap and reinforce or cancel.
- Which-path measurement
- Detecting which slit a particle passed through, which destroys the superposition and the interference pattern.
Watch it explained
- How it worksHow does firing photons one at a time still build a two-slit interference pattern?
- How it worksWatch single electrons land one by one and slowly paint out an interference pattern.
- Common mistakeWhy isn't a photon simply a tiny ball of light traveling along a definite path?
- Going furtherWhat happens to the fringes the moment you put a detector at one of the two slits?
Check yourself
End of topic.