If an object's reflectivity is 0.0 and transmittance is 0.0, what is its emissivity?

• A. 0.0
• B. 0.5
• C. 1.0
• D. 0.8

Answer: (C)

In infrared radiative properties, a surface’s emissivity equals its absorptivity when the surface is at a given temperature and wavelength (Kirchhoff’s law). Energy conservation for a surface says reflectivity plus transmittance plus absorptivity equals 1. If both reflectivity and transmittance are zero, then all incident energy is absorbed, so absorptivity is 1.0. Since emissivity equals absorptivity for a real surface at that condition, the emissivity must be 1.0. This describes a perfect absorber/emitter, like a blackbody, at that wavelength.