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What Is The Difference Between Transmittance And Absorbance

Introduction

When it comes to studying the properties of light passing through a substance, two important terms often come up: transmittance and absorbance. These terms are closely related but refer to different aspects of how light interacts with matter. In this article, we will explore the definitions and differences between transmittance and absorbance.

Transmittance

Transmittance is a measurement of the amount of light that passes through a substance. It quantifies the fraction of incident light that is transmitted through a sample. Transmittance is expressed as a percentage or a decimal value between 0 and 1, where 0 represents no light transmitted and 1 represents complete transmission.

Mathematically, transmittance is calculated using the formula:

Transmittance = (Intensity of transmitted light / Intensity of incident light)

For example, if a substance allows 80% of light to pass through it, the transmittance would be 0.8 or 80%.

Absorbance

Absorbance, also known as optical density, measures the amount of light absorbed by a substance as it passes through it. Unlike transmittance, which quantifies the amount of light transmitted, absorbance quantifies the amount of light absorbed.

Absorbance is determined using a spectrophotometer, which measures the intensity of light before and after passing through the sample. The absorbance value is obtained by measuring the logarithm of the ratio of incident light to transmitted light.

The formula to calculate absorbance is:

Absorbance = -log₁₀(Transmittance)

Absorbance values range from 0 to infinity. A substance with an absorbance of 0 indicates that no light is absorbed, while an absorbance of infinity implies complete absorption.

Relationship between Transmittance and Absorbance

Transmittance and absorbance are inversely related to each other. As transmittance increases, absorbance decreases and vice versa. This relationship is mathematically described by the equation:

Absorbance = -log₁₀(Transmittance)

Thus, if a substance has a transmittance of 0.8, the corresponding absorbance would be:

Absorbance = -log₁₀(0.8) ≈ 0.0969

Similarly, if the transmittance is 0.3, the absorbance would be:

Absorbance = -log₁₀(0.3) ≈ 0.5229

Applications

The distinction between transmittance and absorbance finds applications in various scientific fields, including chemistry, biology, and environmental science:

Chemistry: In chemical analysis, absorbance measurements are commonly used to determine the concentration of a substance in a solution. By measuring the absorbance and applying the Beer-Lambert’s law, one can calculate the concentration of the solute.

Biology: Transmittance is often used in biological assays to assess the concentration or purity of a biological sample, such as proteins or DNA. By comparing the absorbance or transmittance values of a sample with a standard, scientists can determine the characteristics of the biological material being analyzed.

Environmental Science: Transmittance measurements are vital in monitoring water quality, particularly in measuring the presence of pollutants or contaminants. By measuring the absorbance or transmittance of light at specific wavelengths, scientists can detect the presence and quantity of various substances in water bodies.

Conclusion

Transmittance and absorbance are closely related concepts that describe different aspects of how light interacts with matter. Transmittance measures the fraction of incident light that passes through a substance, while absorbance quantifies the amount of light absorbed. These properties have significant applications in various scientific fields, enabling researchers to determine concentrations, assess purity, and monitor environmental conditions. Understanding the difference between transmittance and absorbance is crucial for anyone involved in the study of the behavior of light and matter.

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What Is The Difference Between Transmittance And Absorbance