The classical simplest "problem" is https://en.wikipedia.org/wiki/Rectangular_potential_barrier https://en.wikipedia.org/wiki/Rectangular_potential_barrier An

The classical simplest "problem" is https://en.wikipedia.org/wiki/Rectangular_potential_barrier https://en.wikipedia.org/wiki/Rectangular_potential_barrier And the actual quantum phenomenon is https://en.wikipedia.org/wiki/Quantum_tunnelling https://en.wikipedia.org/wiki/Quantum_tunnelling