# 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} | c_{red} | c_{blue} |

1 | 4000 M^{-1}cm^{-1} | 2000 M^{-1}cm^{-1} | 1e-4 M | 1e-4 M |

2 | 6000 M^{-1}cm^{-1} | 2000 M^{-1}cm^{-1} | 1e-4 M | 1e-4 M |

3 | 6000 M^{-1}cm^{-1} | 6000 M^{-1}cm^{-1} | 4e-4 M | 1e-4 M |

4 | 6000 M^{-1}cm^{-1} | 6000 M^{-1}cm^{-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.

Copyright © 2000 by Brian M. Tissue, all rights reserved.