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  5. Emissivity



Emissivity is the ratio of the amount of energy emitted from an object (radiation) to that from a black body at the same temperature.

The value is between zero and unity (one), and it is closer to zero for polished metal surfaces and white objects, while it is closer to unity (one) for black objects.

Kirchhoff's law states that the emissivity and absorptivity of the same object are equal to each other.