The
penetration of a potential wall by the electron, into regions of negative
kinetic energy, is known as "tunnelling." Classically a particle must have
sufficient energy to surmount a potential barrier. In quantum mechanics,
an electron may tunnel into the barrier (or through it, if it is of finite
width). Tunnelling will not occur unless the barrier is of finite height.
In the example of the H atom, the potential well is infinitely deep, but
the energy of the electron is such that it is only a distance *E*_{n}
from the top of the well. In the example of the electron moving on a line
we assumed the potential well to be infinitely deep regardless of the energy
of the electron. In this case y_{n}
and hence
*P*_{n} must equal zero at the ends of the line
and no tunnelling is possible as the potential wall is infinitely high.