Extinction coefficient refers to several different measures of the absorption of light in a medium:

• In chemistry, the mass extinction coefficient (also called mass attenuation coefficient or mass absorption coefficient) and the molar extinction coefficient are parameters defining how strongly a substance absorbs light at a given wavelength, per mass unit or per molar concentration, respectively.

• In physics, the "extinction coefficient" is the imaginary part of the complex index of refraction, which also relates to light absorption.

(For the quantitative relationship between the chemistry and physics definitions, see Mathematical descriptions of opacity.)

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