Knowing how to calculate absorbance, we can measure the amount of light that is prevented from passing through a given material. To calculate absorbance of a material, you need not know the wavelength of the light that is passing through the material because if you know the intensity of light you can easily calculate absorbance.
To measure absorbance we use the measurement of transmittance of the material. Transmittance of a material is the fraction of light that passes through the material.
- We usually need to measure absorbance of a material while studying analytical chemistry.
- Absorbance of a substance does not have any true units. It is usually written in absorbance units or AU.
- The amount of light absorbed by a material depends upon the material and the intensity of the light. The value of absorbance may vary from 0 to infinity. If the measured absorbance of a material is found to be zero, then the material does not absorb any light.
- If the absorbance value of a substance is 1, then it is concluded that the material absorbs 90 percent of light.
- If the absorbance value is 2 then it means that the material absorbs 99 percent of light.
Calculating absorbance:
The formula for calculating absorbance of a material is given as the logarithmic ratio of intensity of the light after incidence to the intensity of the light before incidence.
A λ = log10 (I/I0)
In the above formula,
- “A λ” is the absorbance of the material through which a light of wavelength λ is passing
- “I” is the intensity of the intensity of the light at a specified wavelength that has passed through the material.
- “I0” is the intensity of the light before it enters the sample material. This is also called as incident light intensity.
Intensity of light is the number of photons passing per second. It is the ratio of power of the light and the energy per photon of the light. Therefore, to measure intensity you need to measure the power of light and get the value for energy per photon of the light. Various instruments can be used to measure the power of light. The most difficult part of calculating the absorbance is the calculation of intensity. Make sure that you have got the exact calculations for intensity of light.
The following procedure will help you in calculating the absorbance of a material:
- Step 1: Take any source of light and a material whose absorbance is to be calculated.
- Step 2: Measure the intensity of the light before you incident light on the source i.e. measure the intensity of light through blank (I0).
- Step 3: Incident light on the material.
- Step 4: Measure the intensity of light when it passes through the substance (I).
Having the above data, you can measure two values, transmittance of the substance as well as absorbance of the substance. Transmittance can be measured by taking the ratio of initial intensity of light and the intensity of light when it passes through the material. When we take log to the base 10 of this, then we are measuring the absorbance.
Transmittance = I0/I
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