## 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 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 Po, 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 Po value does not affect the resulting values of T, %T, or A.

## Self-study Exercises

• Calculate the ratios of Tred/Tblue and Ared/Ablue 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 4000 M-1cm-1 2000 M-1cm-1 1e-4 M 1e-4 M 6000 M-1cm-1 2000 M-1cm-1 1e-4 M 1e-4 M 6000 M-1cm-1 6000 M-1cm-1 4e-4 M 1e-4 M 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.

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