Spectroscopic analysis: Analysis involving the absorption (or emission) of light.

In uv, visible, ir spectroscopy the absorbance of a solution in a spectrophotometric cell (a container with flat transparent walls) is measured.

Absorbance: Symbol A, (), where I_o is the intensity of the light beam entering the solution and I the intensity leaving it.

Beer's law (Beer-Lambert law); . This states that the absorbance, A, of a solution is proportional to the concentration of the absorbing solute, c, and the pathlength, b, of the light, ε is the proportionality constant. (l is sometimes used in place of b.)

Molar absorptivity: The value of ε when the concentation of the solute is measured
in mol L⁻¹. (Extinction coefficient is an older term for this.) The value of ε varies with the wavelength, λ, of the light.