doubt in a formula
Another example of this is taht you also encounter this type of equation in microwave while calculating the VSWR etc. the thing is that you compute that within a material what is the atio of the two quantaties, the maximum and the minumum. this is often used to do calculations about the mismatch in the material.
or in mathematical terms it can show you the relevant shift of the two curves, superimposed over a single medium where they are imposing both constructive and destructive effects. tthe ratio also shows the relative magniyudes of contructive and distructive effect.