Examples of heat transfer by radiation

• The thermos or vacuum flask

A practical vacuum flask is a bottle made of glass, metal or plastic with hollow walls; the narrow region between the inner and outer wall is evacuated of air. It can also be considered to be two thin-walled bottles nested one inside the other and sealed together at their necks.

Using vacuum as an insulator avoids heat transfer by conduction or convection. Radiative heat loss can be minimized by applying a reflective coating(silver) to surfaces. The contents of the flask reach thermal equilibrium with the inner wall; the wall is thin, with low thermal capacity, so does not exchange much heat with the contents, thus does not affect their temperature much.

• The internal silvered surface reflects heat back into the contents.



• The Flask itself consists of a 'double wall' glass vessel.

Heat can only pass through a vacuum by radiation, and radiated heat from outside is being reflected back to the outside by the silvered surface on the outside walls of the glass vessel. In this way, heat transfer into and out of the container (whether hot or cold drinks are contained), will not easily occur. Heat by conduction and convection is also minimised by this construction.