The effects of probe and pumping beam size and modulation frequency on the photoreflectance were investigated for a Silicon wafer considering one and three dimensioal generation and propagation of the thermal and plasma waves, $PR_{1D}$ and $PR_{3D}$. The magnitude of $PR_{1D}$ decreased as the inverse square of the effective beam radius and that of $PR_{3D}$ was 100 times smaller than PR at 0.1um effective beam radius and decreased with the effective beam radius. The phase shift of $PR_{1D}$ was nearly constant at 225˚, whereas that of $PR_{3D}$ increased with the effective beam radius from 0˚ to 225˚. The magnitude and phase of $PR_{3D}$ become the same with those of $PR_{1D}$ by satisfying the equivalence conditions, the probe and pumping beam radii are larger than the thermal and plasma wavelengths, when the effective beam radius was larger than 112um. The $PR_{1D}$ decreased with the modulation frequency as $\omega^{-\frac{1][2}}$, whereas the magnitude of $PR_{3D}$ was nearly constant and 100 times smaller than that of $PR_{1D}$ at 1 kHz modulation frequency. The phase of $PR_{1D$ varied from 180˚ to 225˚, but that of the $PR_{3D}$ increased from 0˚ to that of $PR_{1D}$ with the increase of the modulation frequency. As the modulation frequency increased, the magnitude and phase of $PR_{3D}$ approached to those of $PR_{1D}$ by approaching to the equivalence conditions due to the decrease of the thermal and plasma wavelengths. The good agreements in the modulation frequency dependence of the magnitude and phase of $PR_{3D}$ with those of measured justified three dimension analysis of the photoreflectance.
