Let us consider a beam of identical particles all of which have the kinetic energy E.On both sides of the barrier U = 0,which means that no forces act on the particles there.When a particle of energy E approaches a potential barrier, according to classical mechanics the particle must be reflected.In quantum mechanics, the de Broglie waves that correspond to the particle are partly reflected and partly transmitted, which means that the particle has a finite chance of penetrating the barrier.