Refractive index differences between the layers. Light ray path length differences through the various refractive indices. "Lens" effect of light rays bending differently depending upon the angle of incidence with respect to the curved surface. Any introductory book on optics or light physics will explain the concepts in greater detail.
An excellent source of such information is Konrady's "Applied Optics and Optical Design" This is an old classic that has been reprinted by Dover publications and it is very clear and understandable and covers your topic very well.
