Calculating the Bohr radius or velocity of an excitons

[supercali]

while doing some calculations such as calculating the Bohr radius or velocity of an excitons in semiconductors such as GaAs I didnt understand why is it allowed to use the static permittivity even though the electron and hole aren't as heavy and the spectra is measured at high frequencies such as \omega=10^{15}Hz?

 

[supercali]

anyone ?