Discussion
Which of the following 'medium' is present between the inner and outer layers of a thermos flask and why?
Answer: C
The medium is vacuum, because it prevents loss of heat due to conduction and convection.
A thermos flask has double walls with the air pumped out of the space between them. Conduction and convection both need a material medium, so a vacuum stops heat from passing across the gap by either process. The walls are also silvered to reflect radiation, and the stopper reduces loss through the top.
Air would allow convection and some conduction, so options A and D are wrong. Steel is a good conductor of heat and would carry heat away, so option B is wrong.
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