For an opaque body

Concept:

When thermal radiation falls onto an object,

• The radiation will be absorbed by the surface of the object, causing its temperature to change
• The radiation will be reflected from the surface of the body, causing no temperature change
• The radiation will pass completely through the object, causing no temperature change

Absorptivity (α) is a measure of how much of the radiation is absorbed by the body

Reflectivity (ρ) is a measure of how much is radiation is reflected

Transmissivity (τ) is a measure of how much radiation passes through the object.

Each of these parameters is a number that ranges from 0 to 1.

For any given wavelength (λ), αλ + ρλ + τλ = 1. Objects may have different values of each of these parameters at different wavelengths.

Now,

For an opaque body, transmissivity is 0 since no wavelength is transmitted through it.

∴ For an opaque body sum of absorptivity and reflectivity is 1