Which describes radiation in heat transfer?

• A. Heat transfer by electromagnetic waves emitted by a higher-temperature object and absorbed by a lower-temperature object.
• B. Heat passes through a solid.
• C. Heat transfer by movement of a gas or liquid due to density differences.
• D. Heat transfer by conduction through a liquid.

Answer: (A)

Radiation is heat transfer by electromagnetic waves emitted by a hotter object and absorbed by a cooler one. This mode doesn’t need a medium, so it can occur through vacuum or air, which is why you can feel the Sun’s warmth on your skin even though space has no air. The energy transfer depends on temperature and surface properties; hotter objects radiate more, and surfaces with higher emissivity emit more infrared energy. A practical way to quantify it (in simple cases) uses the idea that power radiated scales with temperature to the fourth power and with surface area and emissivity.

The other descriptions refer to conduction (heat moving by molecular interactions through a solid, or through a liquid) and convection (heat carried by the bulk movement of a fluid due to density differences). These require a material medium and involve either contact or fluid flow, not electromagnetic waves.