candidate: Doing so eventually gives an RMS deviation for the phase offormula_3where formula_4 is the wavelength of the incoming wave , formula_5 is the classical electron radius , formula_6 is the thickness of the " screen , " or the length scale over which the majority of the scattering takes place , formula_7 is the typical size scale of density irregularities , and formula_8 is the root mean squared variation of the electron density about the mean density .