Mechanics of a Thermos Flask:
A thermos flask is designed to minimize heat transfer between the contents and the surroundings through conduction, convection, and radiation.

Design Features:

The Vacuum: The space between the double glass/steel walls is evacuated to create a vacuum. Since conduction and convection both require a material medium (atoms or molecules) to transfer heat, they cannot occur in a vacuum.
Conduction: Heat cannot travel through the vacuum by vibrating particles.
Convection: Heat cannot travel via moving fluid (air/liquid) currents in the absence of a medium.
Silvered Walls: To prevent heat loss by radiation (which can travel through a vacuum), the inner surfaces are silvered to reflect thermal energy back into the flask.

Conclusion: Vacuum is the primary "medium" (or lack thereof) used to block two of the three modes of heat transfer. Air would still allow some convection and conduction as it is a material.
Therefore, option (c) is correct.

S_15 December 2024

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