The spectral response of a silicon solar cell under glass. At short wavelengths below 400 nm the glass absorbs most of the light and the cell response is very low. At intermediate wavelengths the cell approaches the ideal. At long wavelenghts the response fall back to zero. Silicon is an indirect band gap semiconductor so there is not a sharp cut off at the wavelengh corresponding to the band gap (Eg = 1.12 eV).