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'''Absorbance'''

'''Absorbance''', also known as '''optical density''', is a measure of the amount of light absorbed by a sample as it passes through a medium. It is a dimensionless quantity and is commonly used in [[spectroscopy]] to quantify the concentration of a solute in a solution. Absorbance is defined by the [[Beer-Lambert law]], which relates the absorbance to the concentration of the absorbing species and the path length of the sample.

==Principle==
The principle of absorbance is based on the interaction of light with matter. When light passes through a sample, certain wavelengths are absorbed by the molecules in the sample, while others are transmitted. The absorbance (A) is calculated using the formula:

:A = -\log_{10}(T)

where T is the transmittance, defined as the ratio of the transmitted light intensity (I) to the incident light intensity (I_0):

:T = \frac{I}{I_0}

==Beer-Lambert Law==
The [[Beer-Lambert law]] states that absorbance is directly proportional to the concentration (c) of the absorbing species, the path length (l) of the sample, and the molar absorptivity (ε), a constant that depends on the nature of the absorbing species and the wavelength of light:

:A = \epsilon \cdot c \cdot l

This law is fundamental in [[quantitative analysis]] using spectroscopic techniques.

==Applications==
Absorbance measurements are widely used in various fields, including:

* [[Biochemistry]]: To determine the concentration of [[proteins]], [[nucleic acids]], and other biomolecules.
* [[Environmental science]]: To monitor pollutants and contaminants in water and air.
* [[Clinical chemistry]]: To measure the levels of glucose, cholesterol, and other substances in blood samples.
* [[Pharmacology]]: To study drug interactions and concentrations.

==Instrumentation==
The primary instrument used to measure absorbance is the [[spectrophotometer]]. A spectrophotometer consists of a light source, a monochromator to select the desired wavelength, a sample holder, and a detector to measure the intensity of transmitted light. Modern spectrophotometers can measure absorbance across a wide range of wavelengths, from the ultraviolet (UV) to the visible (Vis) and infrared (IR) regions of the electromagnetic spectrum.

==Related Concepts==
* [[Transmittance]]
* [[Reflectance]]
* [[Fluorescence]]
* [[Phosphorescence]]
* [[Absorption spectrum]]
* [[Spectrophotometry]]

==See Also==
* [[Beer-Lambert law]]
* [[Spectroscopy]]
* [[Spectrophotometer]]
* [[Quantitative analysis]]
* [[Molar absorptivity]]

==References==
{{Reflist}}

==External Links==
{{Commons category|Absorbance}}

[[Category:Spectroscopy]]
[[Category:Analytical chemistry]]
[[Category:Physical chemistry]]
[[Category:Optics]]

{{Chemistry-stub}}

Absorbance - Revision history

Prab at 17:09, 18 March 2025

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