Which statement about a blackbody's emissivity is true?

Which statement about a blackbody's emissivity is true?

• A. A blackbody has ε > 1.
• B. A blackbody has ε = 0.
• C. A blackbody has ε = 1; real objects have ε < 1.
• D. A blackbody emits no radiation.

Answer: (C)

Emissivity is a measure of how effectively a surface emits thermal radiation compared with an ideal blackbody, which emits the maximum possible radiation at a given temperature. A true blackbody has emissivity equal to 1, meaning it emits all the radiation it can at that temperature. Real objects, due to surface properties and material non-idealities, have emissivity less than 1. That’s why the statement that a blackbody has ε = 1 and real objects have ε < 1 is the correct description. The other options imply impossible or false scenarios: emissivity cannot exceed 1, ε = 0 would mean no emission at all, and a blackbody certainly emits radiation.

Emissivity is a measure of how effectively a surface emits thermal radiation compared with an ideal blackbody, which emits the maximum possible radiation at a given temperature. A true blackbody has emissivity equal to 1, meaning it emits all the radiation it can at that temperature. Real objects, due to surface properties and material non-idealities, have emissivity less than 1. That’s why the statement that a blackbody has ε = 1 and real objects have ε < 1 is the correct description. The other options imply impossible or false scenarios: emissivity cannot exceed 1, ε = 0 would mean no emission at all, and a blackbody certainly emits radiation.