How does emissivity relate to thermal radiation?

Emissivity is a measure of a material's ability to emit thermal radiation.

In more detail, every object emits thermal radiation, which is energy transferred by electromagnetic waves. This is a result of the thermal motion of charged particles within the object. The amount of thermal radiation emitted by an object depends on its temperature and its emissivity.

Emissivity is a property of a material's surface and is defined as the ratio of energy radiated by a particular material to the energy radiated by a black body at the same temperature. A black body is an idealised physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. It is also a perfect emitter of radiation, and the maximum amount of radiation possible at any given temperature is defined by the black body radiation law.

The emissivity of a material is a dimensionless quantity and is represented by a value between 0 and 1. A material with an emissivity of 1 behaves like a black body, absorbing all incident radiation and emitting the maximum possible amount of thermal radiation for its temperature. Materials with lower emissivity values emit less thermal radiation for a given temperature.

In practical terms, this means that materials with high emissivity, like many metals, are good at both absorbing and emitting thermal radiation. They heat up and cool down quickly. Materials with low emissivity, like polished silver, are poor at absorbing and emitting thermal radiation. They heat up and cool down slowly. Understanding emissivity is important in many areas of physics and engineering, including thermal imaging, insulation design, and climate modelling.