Unless you took care to make allowance for the concentration, you couldn't make any sensible comparisons about which one absorbed the most light.
Suppose then that you wanted to compare this dye with a different compound. However, in an incredibly dilute solution, it may be very difficult to see that it is coloured at all. If it is in a reasonably concentrated solution, it will have a very high absorbance because there are lots of molecules to interact with the light. Suppose you have got a strongly coloured organic dye. The proportion of the light absorbed will depend on how many molecules it interacts with. Just accept that an absorbance scale often runs from zero to 1, but could go higher than that in extreme cases (in other words where more than 90% of a wavelength of light is absorbed).Ībsorbance isn't very good for making comparisons Note: If you don't feel comfortable with logarithms, don't worry about it. In that case, I o/I is 100/I0 (=10) and log 10 of 10 is 1. Log 10 of 1 is zero.Īn absorbance of 1 happens when 90% of the light at that wavelength has been absorbed - which means that the intensity is 10% of what it would otherwise be. The intensities of the sample and reference beam are both the same, so the ratio I o/I is 1. On most of the diagrams you will come across, the absorbance ranges from 0 to 1, but it can go higher than that.Īn absorbance of 0 at some wavelength means that no light of that particular wavelength has been absorbed. A simple bit of maths is then done in the computer to convert this into something called the absorbance of the sample - given the symbol, A.įor reasons to do with the form of the Beer-Lambert Law (below), the relationship between A (the absorbance) and the two intensities is given by: If I is less than I o, then obviously the sample has absorbed some of the light. The intensity of the light passing through the sample cell is also measured for that wavelength - given the symbol, I. This is usually referred to as I o - that's I for Intensity. Everything you need from that page to understand the present topic is repeated below.įor each wavelength of light passing through the spectrometer, the intensity of the light passing through the reference cell is measured.
Note: It isn't essential to read about how the spectrometer works, but you could follow this link if you are interested or if it is on your syllabus.
#GREEK LETTER EPSILON IN LATTICE ENERGY EQUATION SERIES#
If you have read the page about how an absorption spectrometer works, you will know that it passes a whole series of wavelengths of light through a solution of a substance (the sample cell) and also through an identical container (the reference cell) which only has solvent in it. This page takes a brief look at the Beer-Lambert Law and explains the use of the terms absorbance and molar absorptivity relating to UV-visible absorption spectrometry. Absorption spectra - the Beer-Lambert Law
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