In indoor infrared measurements, which statement is true about aluminum surfaces?

• A. They have low reflectivity and high emissivity.
• B. They emit infrared strongly.
• C. They absorb infrared completely.
• D. They have high reflectivity and low emissivity.

Answer: (D)

In infrared measurements, the important idea is how much energy a surface emits compared to how much it reflects. Metals like aluminum, especially when smooth and polished, act like mirrors in the infrared range. Most of the IR radiation that hits the surface is reflected rather than absorbed, so the surface emits only a small amount of IR. Emissivity, which tells you how effectively a surface emits IR, is related to absorptivity by Kirchhoff’s law—low absorption means low emissivity. That’s why aluminum surfaces have high reflectivity and low emissivity. If the surface becomes rough or oxidized, emissivity can increase, but under typical indoor conditions with a smooth aluminum surface, the description is high reflectivity and low emissivity. The other statements contradict this behavior by suggesting the surface emits strongly, absorbs completely, or reflects little, which isn’t characteristic of polished aluminum.