  In this work, we present the results of our multi-band microvariability study
of the famous blazar BL Lac. We performed microvariablity observations of the
source in the optical VRI bands for 4 nights in the year 2016. We studied the
intranight flux and spectral variability of the source in detail with an
objective to characterize microvariability in the blazars, a frequently
observed phenomenon in blazars. The results show that the source often displays
a fast flux variability with an amplitude as large as ~ 0.2 magnitude within a
few hours, and that the color variability in the similar time scales can be
characterized as "bluer-when-brighter" trend. We also observed markedly curved
optical spectrum during one of the nights. Furthermore, the correlation between
multi-band emission shows that in general the emission in all the bands are
highly correlated; and in one of the nights V band emission was found to lead
the I band emission by ~ 20 minutes. The search for characteristic timescale
using auto-correlation function and the structure function analyses reveals
characteristic timescale of ~ 48 minutes in one of the R band observations. We
try to explain the observed results in the context of the passage of shock
waves through the relativistic outflows in blazars.
