The propagation of spatial beams with initially sharp transverse boundaries is investigated theoretically and experimentally with the paraxial wave equation (PWE). The sharp boundaries generate a universal pattern, which is a consequence of the Schrödinger-like nature of the paraxial dynamics. As a consequence, an approximate analytical expression can be derived for the longitudinal propagation dynamics of the beam. Furthermore, it is shown that the validation of the derived analytical approximation is not limited to the zone in which the PWE is valid, but it is valid in the entire space. Therefore, this solution is a good approximation for the solution of the scalar wave equation (and to the Maxwell wave equation whenever the aperture is much wider than the wavelength of light) in the entire space. Good agreement between the analytical expression and experiment results is presented.