Question

The emissivity of a substance is:

• A. Equal to the absorptivity
• B. Less than the absorptivity
• C. Greater than the absorptivity
• D. None of the above

Hint:

In general, the emissivity of a substance can be greater than its absorptivity, especially when the material is a poor absorber of radiation.

Answer 1

The Correct Option is C

The emissivity of a substance refers to its ability to emit energy as radiation. In certain cases, particularly in materials that are good emitters of radiation, the emissivity can be greater than the absorptivity. This is because emissivity refers to the efficiency with which a body emits thermal radiation, while absorptivity is the fraction of incident radiation a body absorbs. For most practical materials, emissivity is greater than absorptivity due to varying efficiency in emission and absorption, especially for non-perfect black bodies.