The x-ray reflectivity of an ultralightweight and low-cost x-ray optic using anisotropic wet etching of Si (110) wafers is evaluated at two energies, C $K_{\alpha }$ $0.28\mathrm{keV}$ and Al $K_{\alpha }$ $1.49\mathrm{keV}$ . The obtained reflectivities at both energies are not represented by a simple planar mirror model considering surface roughness. Hence, an geometrical occultation effect due to step structures upon the etched mirror surface is taken into account. Then, the reflectivities are represented by the theoretical model. The estimated surface roughness at C $K_{\alpha }$ ( $\sim 6\mathrm{nm}\mathrm{rms}$ ) is significantly larger than $\sim 1\mathrm{nm}$ at Al $K_{\alpha }$ . This can be explained by different coherent lengths at two energies.