If the camera angle to the target exceeds 60 degrees, what happens to emissivity and reflectivity?

• A. Emissivity increases and reflectivity decreases
• B. Emissivity decreases and reflectivity increases
• C. Both emissivity and reflectivity increase
• D. Emissivity decreases and reflectivity increases

Answer: (D)

The key idea is that how much a surface emits versus reflects depends on the viewing angle, governed by Fresnel reflectance and Kirchhoff’s law. For most opaque surfaces, emissivity plus reflectivity equals about 1, so if you see more of the surface’s reflection (higher reflectivity) you’re effectively seeing less of its own emitted radiation (lower emissivity).

As the camera angle to the target becomes more oblique (greater than about 60 degrees), the surface behaves more like a mirror. This means reflectivity increases and emissivity decreases. That’s why the correct description is that emissivity decreases while reflectivity increases.

Why the other ideas don’t fit: increasing emissivity while decreasing reflectivity would violate the energy-balance relationship for opaque surfaces, and both increasing at the same time would contradict the fact that more reflection at oblique angles tends to replace emitted energy.