Optical constants of amorphous have been calculated by the Kramers-Kronig analysis in the continuous spectral region including , O K and Si K absorption edges from the reflection spectra measured using synchrotron radiation. It is the first experiment carried out in such a wide energy region with the same sample. It has been found that for extrapolation of the experimental curve in the high energy region, where the photon energy exceeds the ionization potential of the Si K edge, the relation can be used for energies such that at small grazing angles.