ABSTRACT
A thick lens is a physically big lens with spherical surfaces at both ends and separated by a non-negligible distance. A lens system is a nothing but a set of multiple lenses—thin or thick—that are placed apart. When dealing with a single refracting surface and a thin lens, the chapter defines the primary focal point as the point where when a point object is placed, the image is formed at infinity. The primary focal point for a system of refracting surfaces or a thick lens is defined exactly the same way. The equivalent power is the most important quantity associated with a system of refracting surfaces or a thick lens. The equivalent power helps us determine the positions of principal points and the nodal points. The principal planes are conjugate planes, i.e., if the object is on one of the principal planes, the image will be on the other principal plane.
