The smooth-surface optical part of claim 1 wherein the coating comprises a polycondensation composite product of an organometallic compound having a formula represented by R1 M1 (OR2)m wherein R1 and R2 are organic groups having 1 to 6 carbon atoms, M1 is a metal, and m is a natural number, and of at least one organometallic compound having a formula represented by R3 M2 (OR4)m or M3 (OR5)n wherei