The vacuum in a thermoflask reduces heat loss resulting from

Analyze the question: A thermoflask (vacuum flask) has a double-walled glass container with a vacuum between the walls. We need to know which modes of heat transfer the vacuum prevents.

Explain the concept: Heat travels in three ways:

• Conduction needs particles (a material medium) to pass energy from one to the next.
• Convection needs a moving fluid (liquid or gas) to carry heat.
• Radiation is transmitted by electromagnetic waves and needs no medium, so it can cross a vacuum.

Solution:

• A vacuum has no particles, so there is nothing to conduct heat across the gap. Conduction is stopped.
• With no air or fluid, no convection currents can form. Convection is stopped.
• Radiation can still cross a vacuum (just as the Sun's heat reaches Earth through space). The vacuum does not stop it. This is why the inner walls are silvered: shiny surfaces reflect radiation back and reduce radiant heat loss.

So the vacuum reduces heat loss by conduction and convection only. Radiation is reduced by the silvered walls, not by the vacuum.

Checking the other options:

• A: Wrong, because the vacuum does not stop radiation.
• C: Wrong, because radiation passes through a vacuum.
• D: Incomplete, because convection is also stopped.

**Answer: B.

Exam tip: Remember the parts of the flask and what each does:

• Vacuum → stops conduction and convection
• Silvered walls → reduce radiation
• Cork or plastic stopper → reduces conduction and convection through the mouth

Whenever you see "vacuum," think: "no particles, so no conduction or convection, but radiation still passes."