The optical matrix formalism is applied to find optical parameters such as focal distance, back and front focal points, principal planes and the object image equation for a thick spherical lens immerse in two optical media of different indexes of refraction. Then, the formalism is applied to systems compound of two, three and N thick lenses in cascade, immersed in three, four and N+1 optical media of different indexes, respectively. It is found that a simple Gaussian equation is enough to relate object and image distances no matter the number of lenses. This formalism is validated trough a simple optical matrix model of the human eye.

matrix optics, thick lenses, back focal length, front focal length, principal planes, multi-lenses system, human eye