40. Law of Buger-Lambert-Ber. Optical density and transparency of substance.

The Beer-Lambert law relates the attenuation of light to the properties of the material through which the light is traveling. Use of the terms absorbance and molar absorptivity relating to UV-visible absorption spectrometry. For each wavelength of light passing through the spectrometer, the intensity of the light passing through the reference cell is measured. It’s referred to as I₀ - that's I for intensity. If I < I₀, then the sample has absorbed some of the light (neglecting reflection of light off the cuvette surface). The absorbance of a transition depends on 2 external assumptions: 1.The absorbance is directly proportional to the concentration (C) of solution of sample used in the experiment. 2. The absorbance is directly proportional to the length of the light path (l), which is = to the width of the cuvette. Absorbance to conc.: A ∝ c. A=log₁₀(Io/I). A ∝ l . A=log₁₀(IoI)=ϵlc. The constant ϵ is called molar absorptivity or molar extinction coefficient and is a measure of the probability of the electronic transition. Absorbance ranges from 0 to 1, can go higher than that. An abs. of 0 at some wavelength means that no light of that particular wavelength has been absorbed. The intensities of the sample and reference beam are both the same, so the ratio Io/I is 1. Log₁₀ 1=0.

Optical density (OD). Property of transparent substances which refers to its ability to change the path of light. Oil, having a lower physical density than water, nevertheless has a higher optical density. The higher the OD of the medium, the slower light travels in the medium. OD of a substance depends not only on its physical density, but also on its molecular str., ambient conditions such as temperature and pressure. The optical density D is = to the common log of the ratio of the radiation flux F₀ incident on the layer to the flux F that has been attenuated by absorption and scattering in passing through the layer: D = log (F₀/F). So, the optical density is the log of the reciprocal of the transmission coefficient of a layer of a substance: D = log (1/τ). In the definition of natural optical density, the common log is replaced by the natural log. The index of refraction of a substance is a measure of its OD. When a beam of light, traveling in a transparent medium, strikes the surface of a 2nd transparent medium of a different optical density, the beam will split. Part of it will be reflected back into the original medium and the other part will pass on into the new medium as a refracted beam. Refraction is the bending of light rays as they pass from one medium into another medium of different optical density. Laws of refraction: Law 1. When a ray of light passes obliquely from a medium of lesser to one of greater optical density, it is bent toward the normal. Conversely, a ray of light passing obliquely from a denser to a less dense medium is bent away from the normal. Law 2. The refracted ray lies in the same plane as the incident ray and the normal. Law 3. The angle of incidence θ_i and the angle of refraction _r are related as follows: sinθ_i/sinθ_r=v1/v2=n1/n2 where v₁ = velocity of light in Medium1, v₂ = velocity of light in Med 2, n₁ = index of refraction of Med1, n₂ = index of ref of Med2.