  A plane monochromatic wave propagates in vacuum at the velocity of c.
However, wave packets limited in space and time are used to transmit energy and
information. Here it has been shown based on the wave approach that the on-axis
part of the pulsed beams propagates in free space at a variable speed,
exhibiting both subluminal and superluminal behaviours in the region close to
the source, and their velocity approaches the value of c with distance.
Although the pulse can travel over small distances faster than the speed of
light in vacuum, the average on-axis velocity, which is estimated by the
arrival time of the pulse at distances z >> ld (ld is the Rayleigh diffraction
range) and z > ct (t is the pulse width) is less than c. The total pulse beam
propagates at a constant subluminal velocity through the whole distance. The
mutual influence of the spatial distribution of radiation and temporal shape of
the pulse during nonparaxial propagation in vacuum is studied. It is found that
the decrease in the width of the incident beam and the increase in the central
wavelength of the pulse lead to a decrease in the propagation velocity of the
wave packet. It is shown that the velocities of the Bessel-Gauss and
Laguerre-Gauss pulsed beams decrease with increasing orbital angular momentum
(OAM). The results obtained are in good agreement with subluminal and
superluminal measurements in free space.
