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

• In chemistry, the mass extinction coefficient and the molar extinction coefficient are parameters defining how strongly a substance absorbs light, per density or per 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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