A glass composition including, in oxide-based mol %: (a) at least 30% and at most 50% P2O5, (b) at least 25% and at most 65% ZnO, (c) at least 0.1% and at most 10% Al2O3, (d) at least 0% and at most 50% Li2O, (e) at least 0% and at most 50% Na2O, (f) at least 0% and at most 50% K2O, (g) at least 0% and at most 20% MgO, (h) at least 0% and at most 20% CaO, (i) at least 0% and at most 20% SrO, (j) at least 0% and at most 20% BaO, (k) at least 0% and at most 20% SnO, and (l) at least 0% and at most 5% B2O3, at least one of (d) Li2O, (e) Na2O and (f) K2O being more than 0%, and (a)/(b), the ratio of (a) and (b), being at least 0.2 and at most 2.0.