Transfer of heat is thermos is minimised as under:
(i) By conduction: As in conduction heat can transfer by contact of a material medium. In thermos, the air is evacuated between the walls so heat transfer is stopped by conduction mode.
(ii) By convection: As convection mode also requires material (fluid) medium and there is nothing between the walls of thermos so heat does not transfer by connection mode.
(iii) By Radiation: As Ag polish is coated opaque on inner and outer walls of thermos radiation obeys the laws of refraction and reflection so no refraction takes place through opaque wall.
Reflection of outer radiation goes outside of the inner wall goes inside. So the transfer of heat is minimised by polishing.