Radiation basics: Heat transfer by thermal radiation proceeds primarily from a

Introduction / Context:
Thermal radiation is energy emitted by matter due to its temperature. All bodies above absolute zero emit radiation; net exchange between two bodies proceeds from the higher temperature body to the lower temperature one until radiative equilibrium is reached.

Given Data / Assumptions:

• Two surfaces with different absolute temperatures.
• Line-of-sight or participating medium not strongly absorbing (for simplicity).
• View factors and emissivities define the net exchange.

Concept / Approach:
Each surface emits according to its emissive power; the cooler one also emits but less (for similar emissivity and area). Net radiative heat flow equals emitted minus absorbed energy, yielding a direction from hot to cold when temperatures differ.

Step-by-Step Reasoning:

Verification / Alternative check:
The Stefan–Boltzmann relation shows emissive power scales with T^4; higher T produces stronger emission, ensuring net flow from hotter to colder body.

Why Other Options Are Wrong:

• Cold to hot violates the second law for spontaneous processes.
• “Smaller to larger” and “larger to smaller” confuse geometry with thermodynamic direction.
• Transparency/opacity does not set direction; temperature difference does.

Common Pitfalls:
Ignoring that both bodies emit; the net is dictated by temperature and radiative properties, not by size alone.

Final Answer:
hot body to a cold body