Which heat transfer mode occurs in opaque solids?

• A. Radiation
• B. Convection
• C. Conduction
• D. Evaporation

Answer: (C)

Conduction is the primary way heat moves inside opaque solids. In solids, molecules and atoms are tightly packed, so energy passes from hotter regions to cooler ones through direct interactions—think of vibrating atoms transferring energy to their neighbors. In metals, free electrons carry energy rapidly from the hot end toward the cold end and collide with lattice atoms, speeding up the overall heat spreading. In nonmetals, lattice vibrations called phonons do the same job.

Opacity means the material doesn’t transmit radiant energy through its bulk, so you don’t rely on light or thermal radiation traveling inside to move heat from one side to the other. You can still have surface emission or absorption, but the interior heat transfer is governed by conduction. Convection wouldn’t apply here because it requires a fluid that can move, which solids don’t provide. Evaporation is a phase-change process from liquid to gas, not a mechanism for heat transfer within a solid.