Radiative property definition What do we call the ratio of energy absorbed by a body to the total radiant energy incident upon it?

Introduction / Context:
Radiative surface properties include absorptivity (absorptive power), reflectivity, transmissivity, emissivity, and emissive power. Correct definitions avoid serious mistakes in enclosure radiation problems.

Given Data / Assumptions:

• Incident radiant energy is known; fractions are absorbed, reflected, or transmitted.
• Opaque solids typically have negligible transmissivity.

Concept / Approach:
Absorptive power (absorptivity) α = Q_absorbed / Q_incident. Emissive power E is the rate of emission per unit area. Emissivity ε is the ratio of a surface’s emissive power to that of a blackbody at the same temperature.

Step-by-Step Solution:
Identify requested ratio: absorbed/incident.Match to definition: absorptive power (α).Select the corresponding term from options.

Verification / Alternative check:
Energy balance for a surface: α + ρ + τ = 1, where ρ is reflectivity and τ transmissivity.

Why Other Options Are Wrong:

• Emissive power: emission from surface itself, not absorption.
• Emissivity: ratio to blackbody emission, not absorbed/incident.
• Reflectivity: reflected/incident, different quantity.

Common Pitfalls:
Mixing α and ε; they can be equal only under thermal equilibrium and same wavelength band (Kirchhoff’s law), but definitions remain distinct.

Final Answer:
absorptive power