Question

An equiconvex lens of focal length 15cm is cut into 2 halves vertically.What is the focal length of each half?

Answer 1

   focal length of bi-convex lens = 15 cm

     1/f = (μ - 1) [1/R1 - 1/R2]
        1/f   = (μ - 1) 2 / R
      f = R /  [2 (μ-1) ]  = 15 cm

The bi-equi-convex lens will be split into two plano-convex lenses.
   1 / f  = (μ - 1) [ 1 / R - 1/ ∞ ]  = (μ - 1) / R
       f = R / (μ - 1)  = 30 cm

  Hence the focal length of plano convex becomes twice that of bi-convex lens.

Answer 2

hope this will help you