If an object absorbs 30% of incident radiation, what is its emissivity?

• A. 0.30
• B. 0.10
• C. 0.60
• D. 1.00

Answer: (A)

Emissivity and absorptivity are tied together for a surface at a given wavelength: a body that absorbs part of the incident radiation tends to emit radiation at the same fraction, provided the surface is in thermal equilibrium. If the object absorbs 30% of the incoming radiation, its absorptivity is 0.30. For a typical IR problem, we assume the surface is opaque, meaning no radiation transmits through it, so absorptivity equals emissivity. Therefore, the emissivity is 0.30. The other values would require different absorption levels (or a perfect blackbody for 1.00), which isn’t given here.