Hi! I am looking for help with understanding how to specify correct absorbance values for my little study project of modeling attenuation of light in a material. Here are the sources that I am using: 1) The Wikipedia article http://en.wikipedia.org/wiki/Beer-Lambert_law states that the absorbance of a sample A is alpha*l*c, where alpha = (4*pi*k) /lambda and k is the extinction coefficient. Question 1: Is k supposed to be the extinction coefficient of the absorbing species? Question 2: If I am using k to calculate alpha as in the formula above, this makes c to be dimensionless (right?). How shall I then understand c? I found here: http://www.ilpi.com/msds/ref/concentration.html that concentration can be specified as percent by mass or parts per million. Is this the case? Shall the concentration in my case be specified as dimensionless? For example 0.01 for 1 percent of mass of the absorbing species in the material? Thank you for your help!