Answer:
Radiation transfers heat through radiative processes and does not require a medium to take place.
A thermos flask is used to keeping a liquid hot for a long duration of time. But the above processes between thermos flasks and surroundings lead to loss of stored heat. To prevent these processes, the walls of the thermos flask are double layered with a vacuum between them. The vacuum does not allow the heat transfer between the hot liquid inside and the cold air outside. The processes of conduction and convection are unable to occur because of this insulation. This is the basic working principle of a thermos flask by which the contents of the thermos flask remain hot for a long duration of time.
proper explanation is here.