  High energies emissions observed in X-ray binaries (XRBs), active galactic
nuclei (AGNs) are linearly polarized. The prominent mechanism for X-ray is the
Comptonization process. We revisit the theory for polarization in Compton
scattering with unpolarized electrons, and note that the ($k \times
k'$)-coordinate (in which, ($k \times k'$) acts as a $z$-axis, here $k$ and
$k'$ are incident and scattered photon momentum respectively) is more
convenient to describe it. Interestingly, for a fixed scattering plane the
degree of polarization PD after single scattering for random oriented
low-energy unpolarized incident photons is $\sim$0.33. At the scattering angle
$\theta$ = 0 or $\theta \equiv$ [0,25$^o$], the modulation curve of $k'$
exhibits the same PD and PA (angle of polarization) of $k$, and even the
distribution of projection of electric vector of $k'$ ($k'_e$) on perpendicular
plane to the $k$ indicates same (so, an essential criteria for detector
designing). We compute the polarization state in Comptonization process using
Monte Carlo methods with considering a simple spherical corona. We obtain the
PD of emergent photons as a function of $\theta$-angle (or alternatively, the
disk inclination angle $i$) on a meridian plane (i.e., the laws of darkening,
formulated by \citealp{Chandrasekhar1946}) after single scattering with
unpolarized incident photons. To explore the energy dependency we consider a
general spectral parameter set corresponding to hard and soft states of XRBs,
we find that for average scattering no. $\langle N_{sc}\rangle$ $\sim$1.1 the
PD is independent of energy and PA $\sim 90^o$ ($k'_e$ is parallel to the disk
plane), and for $\langle N_{sc}\rangle$ $\sim$5 the PD value is maximum for
$i=45^o$. We also compare the results qualitatively with observation of IXPE
for five sources.
