A formulation of quantum mechanics in which the dual wave–particle nature (see complementarity) of such entities as electrons is described by the Schrödinger equation. Schrödinger put forward this formulation of quantum mechanics in 1926 and in the same year showed that it was equivalent to matrix mechanics. Taking into account the de Broglie wavelength, Schrödinger postulated a wave mechanics that bears the same relation to Newtonian mechanics as physical optics does to geometrical optics (see optics).