The absorption coefficient, α, is related to the extinction coefficient, k, by the following formula: Therefore, a larger number of electrons can interact with the photon and result in the photon being absorbed. As the photon energy increases, not just the electrons already having energy close to that of the band gap can interact with the photon. For photons which have an energy very close to that of the band gap, the absorption is relatively low since only those electrons directly at the valence band edge can interact with the photon to cause absorption. The probability of absorbing a photon depends on the likelihood of having a photon and an electron interact in such a way as to move from one energy band to another. The above graph shows that even for those photons which have an energy above the band gap, the absorption coefficient is not constant, but still depends strongly on wavelength. The magnitude of (epsilon) reflects both the size of the chromophore and the probability that light of a given wavelength will be absorbed when it strikes the chromophore. The absorption coefficient, α, in a variety of semiconductor materials at 300K as a function of the vacuum wavelength of light. The absorption coefficient for several semiconductor materials is shown below. Consequently, this light is not absorbed. Semiconductor materials have a sharp edge in their absorption coefficient, since light which has energy below the band gap does not have sufficient energy to excite an electron into the conduction band from the valence band. The absorption coefficient depends on the material and also on the wavelength of light which is being absorbed. In a material with a low absorption coefficient, light is only poorly absorbed, and if the material is thin enough, it will appear transparent to that wavelength. The absorption coefficient determines how far into a material light of a particular wavelength can penetrate before it is absorbed. Knowing the absorption coefficients of materials aids engineers in determining which material to use in their solar cell designs.Materials with higher absorption coefficients more readily absorb photons, which excite electrons into the conduction band.Different semiconductor materials have different absorption coefficients.Bouguer recognized that this extinction (now often called attenuation) was not linear with distance traveled through the medium, but related by what we now refer to as an exponential function. As light moves through a medium, it will become dimmer as it is being 'extinguished'. The roots of the term absorbance are in the Beer–Lambert law. History and uses of the term absorbance Beer-Lambert law Sometimes the term 'attenuance' or 'experimental absorbance' is used to emphasize that radiation is lost from the beam by processes other than absorption, with the term 'internal absorbance' used to emphasize that the necessary corrections have been made to eliminate the effects of phenomena other than absorption. Light is transmitted through the sample and recorded by the spectrometer. The term absorption refers to the physical process of absorbing light, while absorbance does not always measure only absorption it may measure attenuation (of transmitted radiant power) caused by absorption, as well as reflection, scattering, and other physical processes. In absorption spectroscopy a beam of light is incident on a sample and most of the light passes through the sample. What these uses of the term tend to have in common is that they refer to a logarithm of the ratio of a quantity of light incident on a sample or material to that which is detected after the light has interacted with the sample. While the term has its origin in quantifying the absorption of light, it is often entangled with quantification of light which is “lost” to a detector system through other mechanisms. The term is used in many technical areas to quantify the results of an experimental measurement. Alternatively, for samples which scatter light, absorbance may be defined as 'the negative logarithm of one minus absorptance, as measured on a uniform sample'.

For other uses, see Refractive index, Nucleic acid quantitation, and Neutral-density filter.Ībsorbance is defined as 'the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)'.

Absorption and light intensity equation