**Definition:**The absorption coefficient of a medium, is related to the probability of a photon to be absorbed by the medium after it travels some distance in the medium; more precisely, it is the rate of photon absorption per unit length in the medium. The absorption coefficient depends on medium composition and light wavelength. The inverse of the absorption coefficient, known as the absorption length, can be understood as the distance traveled by a photon before its probability of not being absorbed in the medium has decreased by . Quantitatively, the absorption coefficient can be related with the light intensity attenuation as it traverses the medium. Let represent the intensity at any point of the path, such that the differential intensity is , a differential length along the path of the radiation. Then , and the constant is known as the absorption coefficient.

**Alternative definition:**At any given point in the path of radiation, the Absorption Coefficient determines the fractional reduction in intensity per unit length of the path.

**Synonym:**

**References:**https://doi.org/10.1036/1097-8542.001600

**Related terms:**Absorption