Emissivity describes how closely a real surface matches the emission of a blackbody at the same temperature.

• A. It is defined as the ratio of a real surface emission to a Blackbody emission, with 1 meaning identical
• B. It Is Always 0
• C. It Applies Only To Transparent Surfaces
• D. It Is Independent Of Temperature

Answer: (A)

Emissivity is a measure of how closely a real surface emits radiation compared with a perfect blackbody at the same temperature. It is defined as the ratio of the actual emitted power per unit area from the surface to the emission from a blackbody at that temperature, so a value of 1 means the surface emits exactly like a blackbody. In practice, real surfaces have emissivity less than 1 (they emit less than a blackbody), and emissivity can vary with wavelength, temperature, and viewing angle. This concept applies to real surfaces in general, not just transparent ones, and it is not strictly independent of temperature since the emission characteristics can change with temperature and wavelength.