Abstract: Secondary radiation from the wave front is calculated in the paper with the use of generalised function approach. The Dirac delta impulse ls used as a waveform for both the plane wave and the spherical pressure wave. The results have been obtained analytically thanks to particularly "friendly" features of the operation of convolution with the delta distribution. No approximations have been needed and both solutions are of awaited simple form, i.e.: the original wave displaced forwards - with the proper time delay. Dynamic equilibrium between the elastic and the kinetic aspects of mechanic energy is the only condition of undisturbed propagation of the wave. Every inhomogeneity of the medium reached by the wave breaks the equilibrium. giving birth to a new, scattered wave with features depending on boundary conditions related to the inhomogeneity. The paper gives formal mathematical proof to Fresnel intuitive explanation of the mechanism of forward-only wave propagation with no backward effect.