Beer-Lambert Law Spreadsheet Simulations

Introduction

The Beer-Lambert (Beer's) law and the definitions of transmittance are usually memorized. The attenuation of light as it passes through an absorbing substance is Demonstrating why absorbance is a log function is straightforward and can be connected to radioactive decay and kinetics.

For more details and a related derivation, see the reference document on the Beer-Lambert law.

Beer Lambert Simulation

The spreadsheet displays the following:

The path length of the two curves must be the same since they are plotted on the same chart.

The spreadsheet also has a data entry for P_o, which is listed as the number of photons. This could equally well be light power in watts or light intensity. This parameter is included for added flexibility. The P_o value does not affect the resulting values of T, %T, or A.

Self-study Exercises

• Calculate the ratios of T_red/T_blue and A_red/A_blue for the red and blue results as the following parameters are changed. How do the changes in T and A compare to the changes in the experimental parameters?

	red	blue	cred	cblue
1	4000 M-1cm-1	2000 M-1cm-1	1e-4 M	1e-4 M
2	6000 M-1cm-1	2000 M-1cm-1	1e-4 M	1e-4 M
3	6000 M-1cm-1	6000 M-1cm-1	4e-4 M	1e-4 M
4	6000 M-1cm-1	6000 M-1cm-1	6e-4 M	1e-4 M

Determine the maximum absorbance that can be measured for the following experimental parameters:

• The noise level of an instrument is 0.1% of the power of the light source.
• The noise level of an instrument is 1% of the power of the light source.

Determine the minimum absorbance that can be measured for the following experimental parameters:

• The noise level of the light source is 0.1% of the light power.
• The noise level of the light source is 1% of the light power.