What is the Difference Between Beer’s Law and Lambert’s Law

The key difference between Beer’s law and Lambert’s law is that Beer’s law states that the amount of absorbed light is proportional to the solution concentration, whereas Lambert’s law states that the absorbance and path length are directly proportional.

Beer’s law and Lambert’s law are usually taken in combination with the Beer-Lambert law because they can indicate the relationship of absorbance with both the path length of light inside the sample and the concentration of the sample.

CONTENTS

1. Overview and Key Difference
2. What is Beer’s Law  
3. What is Lambert’s Law
4. Beer’s Law vs Lambert’s Law  in Tabular Form
5. Summary – Beer’s Law vs Lambert’s Law 

What is Beer’s Law?

Beer’s law states that the amount of absorbed light is proportional to the concentration of the solution. This is an equation related to the attenuation of light to the properties of a material. Moreover, this law states that the concentration of a solvent is directly proportional to the absorbance of a solution. Therefore, we can use this relationship to determine the concentration of a chemical species in a solution with the use of a colourimeter or spectrophotometer. Most often, this relationship is useful in UV-visible absorption spectroscopy. However, this law is valid only for solutions with a high concentration.

This law is sometimes known as the Beer-Lambert law, Lambert-Beer law, and as Beer-Lambert-Bouguer law because there are many people were involved in this determination. In other words, more than one law introduced by various scientists is included in Beer’s law. The equation is as follows:

A = εlc 

A – absorbance, ε – molar extinction coefficient, l – length of the path, c – concentration of the solution

However, when Beer’s law states that the amount of absorbed light is proportional to the solution concentration, we need to consider two assumptions in calculating:

1. The path length of the sample is directly proportional to the absorbance.
2. The concentration of the sample is directly proportional to the absorbance.

What is Lambert’s Law?

Lambert’s law states that the absorbance of a sample is directly proportional to the path length of the light inside that sample. Usually, this law is used in combination with Beer’s law, which is then named as Beer-Lambert law. This is because Beer-Lambert law is very useful in spectroscopic analysis other than these individual laws. Lambert’s law was first introduced by Johann Heinrich Lambert.

What is the Difference Between Beer’s Law and Lambert’s Law?

Beer’s law was introduced by August Beer, while Lambert’s law was introduced by Johann Heinrich Lambert. Beer’s law and Lambert’s law are important as a collective equation. The key difference between Beer’s law and Lambert’s law is that Beer’s law states that the amount of absorbed light is proportional to the solution concentration, whereas Lambert’s law states that the absorbance and path length are directly proportional.

The following infographic presents the difference between Beer’s law and Lambert’s law in tabular form.

Summary – Beer’s Law vs Lambert’s Law

In general, Beer’s law and Lambert’s law are usually taken in combination as Beer-Lambert law because they can determine the relationship of absorbance with both the path length of light inside the sample and the concentration of the sample. The key difference between Beer’s law and Lambert’s law is that Beer’s law states that the amount of absorbed light is proportional to the solution concentration, whereas Lambert’s law states that the absorbance and path length are directly proportional.

Reference:

1. Helmenstine, Anne Marie, Ph.D. “Beer’s Law Definition and Equation.” ThoughtCo, Aug. 28, 2020.
2. “Lambert Law.” An Overview | ScienceDirect Topics.

Image Courtesy:

1. “Beer–Lambert law in solution” By Amirber – Own work (CC BY-SA 3.0) via Commons Wikimedia