Refractive-index changes accompanying changes in population of electronic levels of ${\mathrm{Yb}}^{3+}$ ions in a Yb:YAG laser disk under intense diode and laser pumping have been studied by use of both a highly sensitive polarization interferometer and transient grating testing at $633\mathrm{nm}$ . The electronic change of the index that is due to excitation of the ${\mathrm{Yb}}^{3+}$ ions (which have different polarizability of ground state ${}^{2}F_{7∕2}$ and excited level ${}^{2}F_{5∕2}$ ) is strongly predominant over the thermal component. The polarizability differences are $\Delta p\approx (1.9\pm 0.8)\times {10}^{-26}$ and $(1.95\pm 0.25)\times {10}^{-26}{\mathrm{cm}}^3$ as measured at $633\mathrm{nm}$ in interferometric and transient grating experiments, respectively.