Lesson Plan: Beer-Lambert Law and Earth’s Atmosphere

Here is a step-by-step guide to using this lesson plan in the classroom/laboratory. We have suggested these steps as a possible plan of action. You may customize the lesson plan according to your preferences and requirements.

Step 1: Topic introduction and discussion (Go to the Reading)

1. Use chapter reading, ‘The Beer-Lambert Law’ by LibreTexts^TM, to introduce the topic of light transmittance through a medium and Beer-Lambert’s Law.
2. Explain how light transmitted through a medium is affected by its concentration.
3. Also, discuss the importance of the path length for light transmittance through a medium.
4. Use the reading to derive an expression for the Beer-Lambert’s Law.
5. Emphasize on the non-linear (exponential) relation between transmittance and concentration of medium.
6. Thus, define its molar absorptivity or molar absorption coefficient or molar extinction coefficient (ε).
7. Use the examples given in the text, to teach students to use the Beer-Lambert’s Law to calculate values for the concentration, pathlength, and molar absorptivity of a given medium.

Step 2: Extend the understanding (Go to the Reading)

1. Use the reading, ‘Chemical Connections to Climate Change’ by Tom Kuntzleman, Chemical Education Xchange (ChemEd X), to teach students how the Beer-Lambert Law can be used to study the transmittance of sunlight through the atmosphere.
2. Discuss the composition of the atmosphere and list out the gases that make up the atmosphere.
3. Extend the understanding of the Beer-Lambert Law to evaluate the absorbance values of individual gases in the atmosphere.
4. Use the text to initiate a discussion about the absorption potential of greenhouse gases such as carbon dioxide and methane.
5. Discuss how this results in warming on Earth due to the greenhouse effect.
6. Finally, use the reading to explain how this greenhouse gas effect is exacerbated by increasing concentrations of greenhouse gases in the atmosphere.

Step 3: Explore the topic further (Go to the Visualization)

1. Use the simulation, ‘Beer’s Law Lab’ by PhET Interactive Simulations, University of Colorado, to allow students to explore the Beer-Lambert’s Law for absorbance/transmittance of incident light through a solution.
2. Encourage the students to note the absorbance/transmittance values of incident light when the simulation is run.
3. Direct the students to run the simulation for different solution concentrations, pathlengths and wavelengths of incident light.
4. Use the data generated to observe the correlations between these varying parameters (concentration of solution, pathlength and wavelength of incident light) and absorbance/transmittance values.
5. Initiate an enquiry-based discussion for further understanding of these correlations eg how much does the absorbance value change when the concentration of a solution is doubled.

Use the tools and the concepts learned so far to discuss and determine answers to the following questions:

1. What is the Beer-Lambert Law? How is it expressed in an equation?
2. Define the molar absorptivity of a chemical medium.
3. How is the intensity of transmitted light affected by the molar absorptivity of the medium?
4. How does the path length affect the absorbance of transmitted light?
5. How would increasing concentrations of atmospheric greenhouse gases affect the absorbance of sunlight?

The tools in this lesson plan will enable students to:

1. What is the Beer-Lambert Law? How is it expressed in an equation?
2. Define the molar absorptivity of a chemical medium.
3. How is the intensity of transmitted light affected by the molar absorptivity of the medium?
4. How does the path length affect the absorbance of transmitted light?
5. How would increasing concentrations of atmospheric greenhouse gases affect the absorbance of sunlight?

If you or your students would like to explore the topic further, this additional resource will be useful.

Micro-lecture (Video) :

A video micro-lecture, ‘Spectrophotometry introduction’, by Sal Khan, Khan Academy, that gives an introduction of spectrophotometry, transmittance, absorbance, and the Beer-Lambert Law for light incident on a medium.

Go to the Video