  In this paper, starting from some general and plausible assumptions based on
geometrical optics and on a common feature of the truncated Bessel beams, a
heuristic derivation is presented of very simple analytical expressions,
capable of describing the longitudinal (on-axis) evolution of axially-symmetric
nondiffracting pulses when truncated by finite apertures. We apply our
analytical formulation to several situations involving subluminal, luminal or
superluminal localized pulses and compare the results with those obtained by
numerical simulations of the Rayleigh-Sommerfeld diffraction integrals. The
results are in excellent agreement. The present approach can be very useful,
because it can yield, in general, closed analytical expressions, avoiding the
need of time-consuming numerical simulations, and also because such expressions
provide a powerful tool for exploring several important properties of the
truncated localized pulses, as their depth of fields, the longitudinal pulse
behavior, the decaying rates, and so on.
