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# 2.1: Absorptance, and the Definition of a Black Body

If a body is irradiated with radiation of wavelength $$\lambda$$, and a fraction $$a(\lambda)$$ of that radiation is absorbed, the remainder being either reflected or transmitted, $$a(\lambda)$$ is called the absorptance at wavelength $$\lambda$$. Note that $$\lambda$$ is written in parentheses, to mean "at wavelength $$\lambda$$", not as a subscript, which would mean "per unit wavelength interval". The fractions of the radiation reflected and transmitted are, respectively, the reflectance and the transmittance. The sum of the absorptance, reflectance and transmittance is unity, unless you can think of anything else that might happen to the radiation.

A body for which $$a(\lambda) = 1$$ for all wavelengths is a black body.

A body for which a has the same value for all wavelengths, but less than unity, is a grey body.

(Caution: We may meet the word "absorbance" later. It is not the same as absorptance.)