In 1958, Friedel showed that contrary to the well-known classical Thomas-Fermi model of the exponential decay of the charge density around a charged impurity, within the framework of the quantum scattering theory, there occur long-range spatial static oscillations of charge density around the aforementioned impurity. The Friedel oscillations have first been explained using the scattering theory of the electron waves at the localised impurity. Without the impurity, we may assume that the electron wavefunction takes the following form of a plain wave 112 dependent on the wavevector ⃗ k: