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.