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The requirement of a medium distinguishes radiation from the other two heat transfer methods. Option A (Conduction) is incorrect — conduction absolutely requires a material medium. It works by direct contact between particles, so without a medium (like in vacuum), conduction cannot occur at all. Option B (Convection) is incorrect — convection requires a fluid medium (liquid or gas). Convection works through the physical movement of fluid molecules carrying heat from one place to another. Without a medium, convection is impossible. This is why there are no convection currents in space. Option C (Both conduction and convection) is incorrect — as explained above, both conduction and convection require a material medium. Neither can work in a vacuum. Option D (Radiation) is CORRECT — radiation does not require any medium for heat transfer. The image states: निर्वात में ऊष्मा का संचरण किस विधि द्वारा होता है — विकिरण विधि (Radiation) द्वारा (ऊष्मा का संचरण बिना किसी माध्यम के प्रकाश की वेग से सीधी रेखा में होता है). Radiation travels in the form of electromagnetic waves at the speed of light (3 × 10⁸ m/s) in a straight line. It can travel through vacuum. The Sun's heat reaches Earth through 150 million km of space (vacuum) by radiation. For exams: Radiation = no medium needed = can travel through vacuum = travels at speed of light.
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Heat is a form of energy that transfers between objects or systems due to a temperature difference, moving from a hotter body to a cooler one until thermal equilibrium is reached.
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