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What Is Absorptivity?

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  • Written By: Liz Thomas
  • Edited By: C. Wilborn
  • Last Modified Date: 06 December 2016
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Absorptivity refers to the ability of an object or chemical to absorb light or other electromagnetic radiation. Not all substances absorb the same wavelengths of radiation, so something's absorptivity can reveal a great deal about it. This property has many different uses, from chemical identification and quantification, to X-rays and meteorology.

Visible light is part of the electromagnetic spectrum. Each color of light, from violet to red, has a slightly different wavelength, measured in nanometers (nm); visible light ranges from about 380 nm to 740 nm. Electromagnetic waves that are shorter than the violet end of the visible light spectrum include ultraviolet, gamma rays, and X-rays. Infrared, microwaves, and radio waves are on the higher end of the spectrum.

In chemistry, absorptivity is more accurately called molar absorptivity. This is the measurement of how much electromagnetic energy a chemical will absorb, depending on the energy's wavelength. For molar absorptivity, the Beer-Lambert Law plays an important role. This law states that the amount of light that is transmitted or absorbed will depend on the distance the light passes through the chemical, known as the path length, and the concentration of the solution.

The production of X-rays in one example of the use of electromagnetic absorption. Bones and other tissues absorb different amounts of radiation. When X-rays pass through the body, these differences allow an image to be produced.

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The absorptivity of gases in the atmosphere and soil composition has an impact on weather and air temperatures. Gases that have a high absorptivity, such as carbon dioxide, will raise the temperature of air. Compounds in soil will cause light to either absorb or reflect, influencing temperature and weather as well.

A solution's absorptivity can be measured using a spectrometer. This instrument can be set to specific wavelengths. A sample is placed in the machine and exposed to the specified light waves. On the other side of the sample is a detector that measures the amount of light transmitted through the solution. The absorbency of the sample is determined by subtracting the amount of light transmitted from the original light intensity.

The concentration of solutes in a solution can be determined based on this light absorbency. Calibration curves of light absorbency at different concentrations are known or produced for each test. The absorbency of the unknown sample is then compared to the calibration curves. The process is used extensively in chemistry and biological sciences.

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