How does thermos flask work ?

A thermos flask consists of a double walled glass bottle. The space in between the two bottles is evacuated and sealed. Walls are highly polished (i.e silvered) to avoid radiation of heat.The outer polished surface of A does not radiate heat. Even if a little quantity of heat is radiated by A, most of it is reflected back by the polished surface of B. The evacuation of the space in between A & B helps in the elimination of the loss of heat by conduction and convection. Similarly ice kept inside the flask will not melt for a long time because heat radiations from outside cannot enter the inside of the bottle.