Question

using a spectrophotometer what is the relationship between absorbance and the concentration of light absoring substance in a solution.

Answer 1

Absorbance is a term used by chemists. It is the log to base 10 of Io/I. where Io is the intensity of light (or infrare radiation) entering a sample and I is what comes out at a specific wavcelength. It is related to path length (cm), species concentation (moles/lieter) and a species-dependent absorption parameter in liters*mole^-1*cm^-1

As a physicist, I was taught to use the term absorptance, which is somewhat different, I/Io. If the absorptance is A for a light beam travelling through a sample of path length L,

A = 1 - e^(-kL), where
k is the absorption coefficient. k is what is proportional to the concentration of the light-absorbing substance.

In spectroscopy, the absorbance A is defined as

Answer 2

You will find a more complete answer to your question at
http://en.wikipedia.org/wiki/Absorbance