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?