How can I go about determining the lens system matrix for a planar concave thick lens? I analyzed that a light ray first enters the lens and refracts, then it goes through the lens, and then it refracts again from the other side when it leaves. So I need a refraction matrix, transition matrix and a second refracting matrix when the ray leaves the lens. I am having trouble determining these matrices and hence setting up the lens system matrix for a planar concave thick lens. I understand how to do this problem for a bi - convex lens, but the fact that I need to do it for a planar concave lens, is what really confuses me. Can anybody help me?