Multiple Choice
In UV-Vis spectrophotometry, how are percent transmittance (%T) and absorbance (A) related algebraically?
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17. Fundamentals of Spectrophotometry
Measuring Absorbance
Multiple Choice
In UV-Vis spectrophotometry, absorbance is defined as . What are the units of absorbance?
A
Absorbance is dimensionless (no units).
B
Absorbance has units of .
C
Absorbance has units of .
D
Absorbance has units of .
Verified step by step guidance1
Recall the definition of absorbance in UV-Vis spectrophotometry: \(A = -\log_{10}\left(\frac{I}{I_0}\right)\), where \(I_0\) is the intensity of the incident light and \(I\) is the intensity of the transmitted light.
Understand that absorbance is calculated as the logarithm of a ratio of two intensities, \(I\) and \(I_0\), which have the same units (usually watts per square meter or similar).
Since absorbance is the logarithm of a ratio of two quantities with identical units, the units cancel out, making the ratio dimensionless.
The logarithm function operates on a dimensionless number, so the result, absorbance \(A\), is also dimensionless and has no units.
Therefore, absorbance is a pure number without any physical units, which is why it is often described as unitless or dimensionless.
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