potential step
A potential step is the simplest scattering problem: the potential energy holds one value on the left, then jumps abruptly to a different value on the right, like a single stair. A particle approaching from one side meets this sudden change in its energy landscape and must respond. The step is the bare-bones setup for studying how a quantum wave behaves when the rules of its environment change at a sharp edge.
Quantum mechanics gives a surprising answer. A wave hitting the step does not simply pass or simply stop; in general it splits, with part of the wave reflected back and part transmitted forward. This happens even when the particle has more than enough energy to climb the step — a purely wave-like reflection with no classical counterpart, much as light partly reflects off a pane of glass it could easily shine through. The fractions reflected and transmitted depend on the height of the step relative to the particle's energy.
If the step is taller than the particle's energy, classical physics says the particle must bounce straight back. Quantum mechanics agrees that, in the long run, it is fully reflected, yet the wavefunction does not vanish instantly at the edge: it pokes a short, exponentially fading distance into the forbidden region beyond. That small penetration is the same effect that, in a thin-enough barrier, becomes tunnelling.
At the edge an incoming wave partly reflects and partly transmits — even when it has enough energy to pass.
Quantum reflection from a step a particle could classically surmount is not a measurement error or a trick of timing; it is a genuine wave effect, with the same origin as the partial reflection of light at a surface.