Thermal radiation depends on surface emissivity

Steps:

• Boiling water heats the metal cube, causing vertical faces to emit thermal radiation outward.
• Emissivity (ε) measures radiation efficiency; ranges from 0 (no emission) to 1 (perfect blackbody).
• Thermometers at equal distances detect air temperature warmed by radiation from the nearest face.
• Highest ε yields most radiation, raising nearby temperature most.

Why A is correct:

• Dull black has highest emissivity (ε ≈ 0.95), emitting maximum thermal radiation per Stefan-Boltzmann law (P = εσAT⁴).

Why the others are wrong:

• B. Dull white: Lower emissivity (ε ≈ 0.9) than black, emits less radiation.
• C. Shiny black: Polish reduces emissivity (ε ≈ 0.8) compared to dull black.
• D. Shiny white: Lowest emissivity (ε ≈ 0.2), reflects most heat, emits least.

Final answer: A