A biconvex lens with its two faces of equal radius of curvature R is made of a transparent medium of refractive index 
1. It is kept in contact with a medium of refractive index 2 as shown in the figure.
a) Find the equivalent focal length of the combination.
b) Obtain the condition when this combination acts as a diverging lens.
c) Draw the ray diagram for the case 
when the object is kept far away from the lens. Point out the nature of the image formed by the system.
a) Using the Len’s maker’s formula, we have
Let feq is the equivalent focal length of the combination, then
b)
For, the combination of lenses to behave as diverging lens, equivalent focal length < 0.
c)
For 
the combination of lenses will behave as converging lens. An object placed far from the lens will form image at the focus of the lens.
Image formed is real and diminished in nature.
