Infra red absorption study has been carried out on a series of half doped manganites, La_0.5Ca_0.5__x Sr_xMnO₃ (0:1≤x≤0:5),  with varying magnetic ground state. The charge ordering transition observed in  samples with x≤0:3 is accompanied by a mode at ~ 525 cm^-1 in addition to the stretching mode at 615 cm^-1 and bending mode at 400 cm^-1. Phonon hardening is found to occur below the CE-type anti ferromagnetic ordering temperature. The value of the insulating gap decreases on doping with Sr from 727 to 615 cm^-1.