TABLE OF CONTENTS In the case of refraction through a thin lens, the position and size of the image formed is determined by neglecting the thickness of the lens, as it is very small compared to the distances of the objects and images from it. But in the case of a thick lens or coaxial system of lenses, we cannot proceed with this assumption. In addition, the method of finding the position and size of the final image by considering refraction at each surface of a lens successively, is extremely tedious and complex. To overcome this difficulty, in 1941, Gauss proved that the positions of certain specific points are known. These specific points are called cardinal points. The pair of the points form six cardinal points of an optical system; two focal points, two principal points and two nodal points. All the six points are situated on the optical axis of the system and are conjugate to one another.
Figure 1 shows the combination of two thin convex lenses, L 1 and L 2, separated by a distance d apart. Let a ray of light OA, parallel to the axis is incident on first lens L 1 at a height h 1 The lens L 1 deviates this ray along AB through an angle ? 1. The refracted ray AB strikes lens L 2 at a height h 2 and further deviates through an...
