Marisol has one green laser. At two narrow slits, its light makes bright and dark fringes on a screen. Aimed at a metal whose threshold frequency sits below green light's, the same beam ejects electrons, one photon per electron. The same beam does both.
The last two lessons sorted the evidence: interference and diffraction on the wave side, the photoelectric effect on the particle side. This lesson says what that sorting means for light itself.
Light behaves like a wave in some situations. At two slits, it spreads, overlaps, reinforces and cancels.
Light behaves like a stream of particles in other situations. At a metal surface, it delivers energy in separate whole lumps, one photon per ejected electron.
Neither model alone covers all the evidence. The wave model cannot make a threshold frequency. The photon model's separate lumps cannot cancel each other out to make dark fringes.
So physics keeps both models and uses each where it works.
Wave-particle duality does not mean light stops being one thing. Both pictures are needed, because each explains behaviors the other cannot.
The same light that forms interference fringes at two slits also ejects electrons one photon at a time at a metal. That one sentence is wave-particle duality in action.
Kenji's ultraviolet lamp makes a fringe pattern at two slits in one experiment and ejects electrons from zinc in another. Which model of light does each experiment call for?
The fringe pattern is a stable set of reinforced and canceled bands, and only overlapping waves cancel. The first experiment calls for the wave model.
The ejection from zinc runs one photon per electron, in whole lumps. The second experiment calls for the photon model.
The light is the same in both experiments, so neither model alone covers it.
The first experiment calls for the wave model, the second for the photon model: the lamp's light shows wave-particle duality.
A student argues that the fringe experiment proves the photon model is wrong. What does the evidence actually support?
The fringe experiment shows a behavior the wave model explains and the photon model does not.
The photoelectric effect shows a behavior the photon model explains and the wave model does not.
Evidence against one model in one situation does not erase the evidence for it in another.
The evidence supports keeping both models: light shows wave-particle duality.
Why do physicists keep both the wave model and the photon model of light?
Jocasta says: 'No single model of light explains every behavior light shows.' Is Jocasta correct?
Light behaves like a wave in some situations and like a stream of particles in others. What is this double behavior called?
Which statement is wave-particle duality?
One violet lamp's light spreads out after a very narrow slit, and it also ejects electrons from a potassium surface. What do the two results together show?
A student claims the photoelectric effect proves light is only a stream of particles. What is wrong with the claim?