Consider a convex lens (or concave lens) of absolute refractive index to be placed in a rarer medium of absolute refractive index . Considering the refraction of a point object on the surface , the image is formed at who is at a distance of .
(as the lens is thin)
It follows from the refraction due to convex spherical surface
The refracted ray from A suffers a second refraction on the surface and emerges along BI. Therefore I is the final real image of O.
Here the object distance is
is very small
(Final image distance)
Let be radius of curvature of second surface of the lens. It follows from refraction due to concave spherical surface from denser to rarer medium that
Thus we get=