[填空题]
Two (and only two) full spectral orders cdr 9v top lr:cgjxz+duso7w ukr.1(1(wwz87:an be seen on either side of thett;sya x:4 *yn f(d,t+ ab6ktf central maximum when white light is sent through a diffraction grating. What is the maximum number of la4 b, : y *stxty+t(;k6aftfdnines per cm for the grating? $ \times10^{ 3}\;lines/cm$
