# Absorption

Attenuation of the radiant flux when light is passed through a clear medium.

If light of a suitable wavelength is passed through a sample, part of the energy is transmitted to the molecules. As a result, the emergent beam Φout has less energy than the incident beam Φin Fig. 2: Absorption

The amount of light absorbed generally follows the Lambert-Beer Law and is therefore proportional to the number of absorbing molecules and the path length traversed L. Absorption spectroscopy; Extinction

If the medium contains turbidity matter, additional attenuation is caused as a result of light scattering. Scattered light

Terms related to absorption spectroscopy:
Lambert-Beer Law: $\phi_{\text{out}} = \phi_{\text{in}} \cdot 10^{- \kappa \cdot c \cdot L}$
Absorbance:
(previously referred to as "extinction E")
$A = \log_{10} \left( \frac{ 1 }{ \tau } \right) = \log_{10} \left( \frac{\phi_{\text{in}}}{\phi_{\text{out}}} \right)$
Absorption coefficient: $a = \left(\frac{A}{L}\right) \quad a = \kappa \cdot c$
Transmittance: $\tau = \left(\frac{ \phi_{\text{out}} }{ \phi_{\text{in}} }\right)$
$\phi_{\text{out}}$ Radiant flux (or radiant power), transmitted
$\phi_{\text{in}}$ Radiant power, received
$\kappa$ Relative absorption coefficient (previously referred to as "extinction coefficient ε")
$c$ Concentration in mol/l
$L$ Path length in cm
Index

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