Concept:In a thermos flask, heat transfer by radiation is reduced by reflecting thermal radiation back toward its source.
Explanation:Heat loss by radiation occurs in the form of infrared waves that can travel across empty space.
The thermos flask has double glass walls that are silvered on their inner surfaces.
Silvered surfaces are highly reflective and are also poor emitters of radiation.
When heat tries to radiate outward from the hot liquid, the silver coating reflects the heat back into the contents of the flask.
Thus, the silvered walls prevent radiant heat from escaping through the walls.
The vacuum prevents conduction and convection, while the plastic stopper and cork support reduce conduction losses, but they do not minimise radiation.
Therefore, the feature that directly minimises radiation loss is the silvered surfaces.
Answer:A. silvered surfaces