  We studied the doping and temperature (T) dependence of the infrared (IR)
response of Ba(Fe1-xCox)2As2 single crystals. We show that a weak band around
1000 cm-1, that was previously interpreted in terms of interaction of the
charge carriers with magnetic excitations or of a pseudogap, is rather related
to low-energy interband transitions. Specifically, we show that this band
exhibits a similar doping and T-dependence as the hole pockets seen by angle
resolved photoemission spectroscopy (ARPES). Notably, we find that it vanishes
as a function of doping near the critical point where superconductivity is
suppressed in the overdoped regime. Our IR data thus provide bulk specific
information (complementary to the surface sensitive ARPES) for a Lifshitz
transition. Our IR data also reveal a second low-energy band around 2300 cm-1
which further emphasizes the necessity to consider the multiband nature of
these iron arsenides in the analysis of the optical response.
