  In this presentation, we report the results of intraday variability in the
optical (BVRI bands) and hard X-ray band (3-79 keV) in a number of blazars. In
the optical microvariability studies of the blazars S5 0716+714 and BL Lac, we
observed many interesting features such as rapid variability, large variability
amplitude, presence of characteristic timescales, bluer-when-brighter
achromatic behavior, and single power-law power spectral density. In {\it
NuSTAR} observations of several blazars, using spectral and timing analysis, we
found similar features consistent with the optical studies. In addition, in BL
Lacs we estimated the Lorentz factor of the population of highest energy
electrons emitting synchrotron emission, and whereas in flat-spectrum radio
quasars, using external Compton models, we estimated the energy of the lower
end of the injected electrons to be a few tens of Lorentz factors. In addition,
we find that the low flux state exhibit more rapid variability in contrast to
the previously reported results showing high flux states displaying rapid
variability. In both the studies, the size of the emission regions estimated
using variability timescales turn out to be an order magnitude smaller than the
gravitational radius of a typical black-hole masses between $\sim 10^8-10^9$
solar masses, believed to be at the center of the radio-loud AGN. The results
of the studies suggest that these low-amplitude rapid variability might
originate as a result of magnetohydrodynamical instabilities near the base of
the jets triggered by the processes modulated by the magnetic field at
accretion disc.
