  Observations of redshifted 21-cm radiation from neutral hydrogen during the
epoch of reionization (EoR) are considered to constitute the most promising
tool to probe that epoch. One of the major goals of the first generation of low
frequency radio telescopes is to measure the 3D 21-cm power spectrum. However,
the 21-cm signal could evolve substantially along the line of sight (LOS)
direction of an observed 3D volume, since the received signal from different
planes transverse to the LOS originated from different look-back times and
could therefore be statistically different. Using numerical simulations we
investigate this so-called light cone effect on the spherically averaged 3D
21-cm power spectrum. For this version of the power spectrum, we find that the
effect mostly `averages out' and observe a smaller change in the power spectrum
compared to the amount of evolution in the mean 21-cm signal and its rms
variations along the LOS direction. Nevertheless, changes up to 50% at large
scales are possible. In general the power is enhanced/suppressed at large/small
scales when the effect is included. The cross-over mode below/above which the
power is enhanced/suppressed moves toward larger scales as reionization
proceeds. When considering the 3D power spectrum we find it to be anisotropic
at the late stages of reionization and on large scales. The effect is dominated
by the evolution of the ionized fraction of hydrogen during reionization and
including peculiar velocities hardly changes these conclusions. We present
simple analytical models which explain qualitatively all the features we see in
the simulations.
