The emissivity, reflectivity, and transmissivity of a surface must add up to what value?

The emissivity, reflectivity, and transmissivity of a surface must add up to what value?

• A. 0
• B. It varies
• C. 1
• D. 2

Answer: (C)

Radiant energy at a specific wavelength and direction is split into three fractions: what the surface emits (emissivity), what it reflects (reflectivity), and what it transmits (transmissivity). Energy conservation requires that the sum of these three fractions equals 1. So the total is 1. In many infrared situations the surface is opaque, making transmission negligible, which means emissivity plus reflectivity account for nearly all the energy: ε + ρ ≈ 1. The other options would imply missing or extra energy, which isn’t allowed by energy conservation.

Radiant energy at a specific wavelength and direction is split into three fractions: what the surface emits (emissivity), what it reflects (reflectivity), and what it transmits (transmissivity). Energy conservation requires that the sum of these three fractions equals 1. So the total is 1. In many infrared situations the surface is opaque, making transmission negligible, which means emissivity plus reflectivity account for nearly all the energy: ε + ρ ≈ 1. The other options would imply missing or extra energy, which isn’t allowed by energy conservation.