  Whether gamma-ray bursts are highly beamed or not is a very difficult but
important problem that we are confronted with. Some theorists suggest that
beaming effect usually leads to a sharp break in the afterglow light curve
during the ultra-relativistic phase, with the breaking point determined by
$\gamma = 1 / \theta_0$, where $\gamma$ is the Lorentz factor of the blastwave
and $\theta_0$ is the initial half opening angle of the ejecta, but numerical
studies tend to reject the suggestion. We note that previous studies are
uniformly based on dynamics that is not proper for non-relativistic blastwaves.
Here we investigate the problem in more detail, paying special attention to the
transition from the ultra-relativistic phase to the non-relativistic phase. Due
to some crucial refinements in the dynamics, we can follow the overall
evolution of a realistic jet till its velocity is as small as $\beta c \sim
10^{-3} c$. We find no obvious break in the optical light curve during the
relativistic phase itself. However, an obvious break does appear at the
transition from the relativistic phase to the Newtonian phase if the physical
parameters involved are properly assumed. Generally speaking, the Newtonian
phase is characterized by a sharp decay of optical afterglows, with the power
law timing index $\alpha \sim 1.8$ - 2.1. This is due to the quick lateral
expansion at this stage. The quick decay of optical afterglows from GRB 970228,
980326, and 980519, and the breaks in the optical light curves of GRB 990123
and 990510 may indicate the presence of highly collimated $\gamma$-ray burst
ejecta.
