# Molar absorption coefficient - why are the molar absorptivities defined to be the same?

In spectroscopy, the isosbestic point is said to be where the molar absorptivities of two species is the same.

$$A= \epsilon(X+Y) \times c(X+Y) \times l$$

reduces to $$A= \epsilon_t \times c \times l$$

the absorbances and the length are the same at this point, but the concentrations are not;

why are the molar absorptivities defined to be the same?

Is c taken to be the relative contribution, rather than the exact concentration?

• Please write descriptive titles. – M.A.R. ಠ_ಠ Mar 4 '15 at 19:48
• I'd rather say the definintion is the other way round: the point where no change in absorbance was found during the reaction got a new name. – cbeleites Mar 4 '15 at 21:56