In a thermos flask, heat loss by radiation is minimizes by the

Analyze the question: A thermos (vacuum) flask is built to reduce heat transfer by conduction, convection and radiation. The question asks which feature minimizes heat loss by radiation specifically.

Explain the concept:

• Conduction and convection need a material medium. A vacuum between the double walls removes the medium, so these are minimized.
• Radiation (infrared waves) can travel through a vacuum, so the vacuum does not stop it. It is reduced by shiny, silvered surfaces, which are poor emitters and good reflectors of radiant heat. Hot contents therefore reflect heat back inward, and cold contents reflect outside heat away.
• The plastic stopper and cork support are poor conductors. They reduce heat loss by conduction at the neck and base.

Matching each feature to the heat transfer it reduces:

| Feature | Heat loss reduced | |---|---| | Silvered surfaces | Radiation | | Vacuum | Conduction and convection | | Plastic stopper | Conduction (and convection through the opening) | | Cork support | Conduction |

Answer: The silvered surfaces minimize heat loss by radiation.

Exam tip: Remember "Silver = Shine back radiation" and "Vacuum = no particles, so no conduction or convection." In WASSCE and JAMB, if a question asks about radiation in a flask, think of the silvered walls first.