What is the difference between absorbance and transmittance?
Transmittance and absorbance are two key concepts mainly used in analytic and spectrometry chemistry. These two terms tend to give learners a lot of problems when it comes to revision for exams.
The discussion provides the core difference between absorbance and transmittance with a comparison chart for easier understanding. Besides that, we have provided a comparison video to facilitate an easier understanding of the concepts.
What Is Absorbance?
Absorbance is a technique used to measure the amount of light that travels through a material. However, getting the exact value when using the absorbance technique is quite difficult since the values tend to be less than one.
The formula for calculating absorbance is A= Log10 = () besides that, you can use the percentage formula to attain the same answer A= 2- log10 (% T)
The Beer-Lambert Law states that the absorbance of light passing through a solution is directly proportional to the path length of light through the material (l) and the concentration (c). The law can be written in form of A=€lc. € represents a constant value known as molar absorptivity.
The value of molar absorptivity tends to be specific for a given substance regardless of the temperature of the substance and the wavelength of light remains constant. This will help to get to know the relationship between concentrations of an unknown solution.
The instrument used to measure the absorbance of light in the laboratory is known as a spectrophotometer. This can help you draw a graph showing the relationship between concentrations and their absorbance.
What Is Transmittance?
Transmittance is a technique that is used to measure how much light passes through a substance. The larger the transmittance, the higher the amount of light that passes through a substance and vice versa.
Typically, transmittance describes the ratio between the intensity of incident light and the intensity of transmitted light. The intensity of incident light is denoted as I0 and intensity of transmitted light as I. Therefore, the formula of transmittance is stated as T=I/I0. The fraction can be written as a percentage such as (% T)
Comparison Chart: Absorbance Vs Transmittance
| Basic Terms | Absorbance | Transmittance |
| Meaning | Refers to the amount of incident light traveling through a material | This refers to the measure of how much light passes through a substance. |
| Formula | A= Log10 = () | T=I/I0 |
| How the Value Changes as the Path Length/Concentration is Increased | Decreases exponentially | Increases linearly |
| Range | Values range from 0 to 1. | Could take values from 0 upwards. |
| Instrument | Spectrophotometers | Spectrophotometer |
Core Difference between Absorbance and Transmittance
- Transmittance directly measure the quantity whereas absorbance calculate the transmittance measurement
- Transmittance measure how much light passes through a sample whereas absorbance show amount of light absorbed by the material
- Absorbance value range between zeros to one while transmittance value range from zero to upward.
- The value absorbance decreases exponentially while transmittance increases linearly when the value changes as the path length or concentration is increased.
- The formula of absorbance is A= Log10 = () while that of transmittance is T=I/I0
Read More: Difference between Photosystem I and Photosystem II
Comparison Video
Summary
The core difference between absorbance and transmittance is that absorbance describes how much incident light is absorbed when light travels through the material whereas transmittance describes how much light passes through a sample.