The growth mode of Si1-xGex thin films on Si (1 0 0) can be judged in situ by simply monitoring the multiwavelength psi-Delta trajectories measured by spectroscopic ellipsometry. During the two-dimensional Si0.65Ge0.35 growth the trajectory obtained at 3.4 eV (an energy where the Si0.65Ge0.35 is absorbing) quickly converges to a point that defines the dielectric constant of the material, but the trajectory obtained at 2.3 eV (an energy where the Si0.65Ge0.35 is semi-transparent) is a spiral asymptotically approaching a virtual convergence point. During the Stranski-Krastanov growth, on the other hand, the trajectory at 3.4 eV is initially close to the trajectory monitored during two-dimensional growth but transferred to a different branch beyond the critical thickness. When psi-Delta trajectory is monitored at 2.3 eV, the virtual convergence point shifts toward lower II, values as growth proceeds.