text: what is the name of the medium between a convex/concave lens? answer is not glass or plastic. think optic geometry or anatomy. need a clarification: do you mean between the first and second surfaces of a single lens, or do you mean between two separate lenses like the two components of an achromat doublet or between the concave and convex components of an afocal zoom lens?  still seeking clarification of your question. do you mean the blue stuff in the second image i linked or the white stuff in between the blue stuff?