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Beer-Lambert Law of Spectroscopy

In chemistry, the absorption of light by a specific type and collection of molecule is defined quantitatively by three simple factors

The Beer-Lambert Law of Spectroscopy relates the three main factors that control the amount of light of a given wavelength that can pass from the source to the phototube of a spectrophotometer, or for that matter, the eye of an observer.  It is true everywhere on earth, and presumably, everywhere in normal space.  It assumes that no other perturbations or other variables are present, such as optically interfering substances (e.g., light-scattering particles or transparent, refractive bodies). 
The three factors are
1.  A, the Absorption Coefficient,
2.  C, the concentration of the absorbing species,
3.  L, the pathlength through which the ray of light passes from source to detector.
Thus, Beer-Lambert's relationship states that the amount of light absorbed along its path is directly proportional to A, C and L.
Each chemical compound has its own characteristic set of Absorption Coefficients.  The are wavelength-specific and are ultimately determined by the electronic energy levels that are possible from that compound's molecular structure. 
The greater the value of the Absorption Coefficient at a given wavelength, the more electromagnetic energy at that wavelength is absorbed when light passes through a dispersed population of that molecule (i.e., "darker colored" versus "weakly colored").
The higher the concentration of absorbing particles within the light path, the more light is absorbed.
The longer the pathlength (i.e., the farther the light source is from the observer), the more light is lost through absorption.

Elemental carbon is black in color.  What that means is that it completely absorbs light AT ALL WAVELENGTHS in the visible spectrum!  That is the first prerequisite for a universal fluorophore, that it can absorb any visible wavelength.  That capability is controlled by the extended system of conjugated carbon-carbon Pi bonds that form a continuously variable path for electron movements between the carbon atoms throughout the molecule.  These Pi electrons can move freely around the molecular bonds as if they were copper wires.  In this kind of electronic structure, the primary absorbance energy (i.e., Lambda max, the maximum wavelength of absorption) is determined by the maximum pathlength within the molecular structure that the electrons can move.  The shorter the molecular pathlength, the more energetic the absorption spectrum for that compound (e.g., the closer the Lambda max is to the UV end of the electromagnetic spectrum).  Molecules with longer conjugated systems of Pi bonds have their Lambda max shifted towards the red end of the spectrum.  Remember, this is the first step of the fluorescence phenomenon.   
Once a molecule absorbs Ultraviolet or visible light energy, it causes a change in the movement of these Pi electrons between the bonds in such a compound.  This excess energy ultimately is lost spontaneously as the excited state of the molecule attempts to return to its ground state, or "original lowest energy" configuration.  For carbon-based molecules, such as polyaromatic hydrocarbons (i.e., PAHs), these compounds generally get rid of their excess electron energies by undergoing a complex set of changes in their mobil Pi electron motions.  The energy can be released all at once (not very probable) and thus results in a reemission of a photon of the same wavelength as the one that was absorbed.  But the most likely return to the ground state involves a multistep cascade of electron energy releases, each emitting a quantum of light of a different, and smaller energy content than the original light energy that excited the molecule in the first place.  Thus a higher energy photon of light is absorbed, and then is broken down into a number of photons of lower energy contents during their reemission to the outside world.