Emissivity is defined as the ratio of the emissive power of a real surface to that of an ideal black body at the same temperature. Is this statement correct?

Introduction / Context:
Real surfaces emit less thermal radiation than an ideal black body at the same temperature. Emissivity quantifies this deficit and is a key property in furnace design, infrared thermography, and radiation heat transfer analysis.

Given Data / Assumptions:

• E is emissive power of the real surface.
• E_b is emissive power of a black body at the same temperature.
• Emissivity ε is defined between 0 and 1.

Concept / Approach:
The standard definition is ε = E / E_b. A black body has ε = 1. Highly reflective or polished surfaces have low ε. This definition applies generally, independent of environment (vacuum or presence of a gas) though surroundings affect net exchange and view factors.

Step-by-Step Solution:

Verification / Alternative check:
Heat transfer handbooks consistently define emissivity in these terms; spectral and directional variants exist, but the basic total hemispherical definition remains the ratio described.

Why Other Options Are Wrong:

• Restrictions to metals, vacuum, or very high temperatures are unnecessary; emissivity is broadly defined for all surfaces and temperatures.

Common Pitfalls:
Confusing emissivity with absorptivity or reflectivity; while related via Kirchhoff’s law under thermal equilibrium, the definitions are distinct.

Final Answer:
Agree