Heat transfer that does not require a medium is called
Radiation
Radiation is the only mode of heat transfer that needs no material medium — option 1.
| Mode | Medium needed ? | Mechanism | Typical example |
|---|---|---|---|
| Conduction | Yes — solids mainly | Vibration of molecules and, in metals, drift of free electrons; no bulk movement of matter | Spoon handle heating in hot tea |
| Convection | Yes — fluids only | Actual movement of heated fluid, since warm fluid expands, becomes less dense and rises | Boiling water, sea breeze |
| Radiation | No | Electromagnetic waves, chiefly infrared, which travel through vacuum | Heat of the Sun reaching the Earth |
The decisive proof. Between the Sun and the Earth lie about 150 million kilometres of near-perfect vacuum with no matter to conduct or convect through, yet the Earth receives roughly 1,361 watts per square metre of solar energy at the top of the atmosphere. Energy therefore crosses empty space, and it can only do so as electromagnetic waves.
The physics. Every body above absolute zero radiates energy. The Stefan-Boltzmann law gives the power radiated per unit area of a black body as
\(E=\sigma T^{4}\)
so a modest rise in temperature increases radiated power sharply, and Wien’s displacement law tells us that hotter bodies radiate at shorter wavelengths — which is why the Sun’s output peaks in visible light while a room-temperature object radiates in the infrared.
Why reflection is not an answer. Reflection is not a mode of heat transfer at all; it is what happens when radiation strikes a surface and bounces back instead of being absorbed. It redirects energy rather than transporting it, which is why a polished silver surface is a poor radiator and a poor absorber alike — the principle behind the vacuum flask, which blocks all three modes at once with a vacuum jacket and silvered walls.
Hence, the answer is Radiation.
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Important Questions from Heat and Transfer
In summer season, the top and outer surface of the house are painted white because:
Which of the following is a process in which hot, less dense materials rise upward and are replaced by colder, more dense materials?
What is the heat released by a heating filament rated 24 W when it is kept on for 6 seconds?
Space is left between logs of wood when they are burnt, so that:
On a hot summer afternoon, the tent made up of fabric will be preferred as white colour is:
