Question

what happens if the lens is thick or thin

Answer 1

in optics, a thin lens is a lens with a thickness (distance along the optical axis between the two surfaces of the lens) that is negligible compared to the radii of curvature of the lens surfaces. Lenses whose thickness is not negligible are sometimes called thick lenses.

Answer 2

when a  lens is thick or thin there is a change in the focal length of the lens
when the lens is thick the lens has a shorter focal length
when the lens is thin it has a large focal length