Question

An equi convex lens of focal length 2 m then it cut to two plano-convex lens then what is its focal length. ?

Answer 1

Let the focla legth of the equiconvex lens be f and of each identical plano convex lens be f'

For the equiconvex lens, the focal length is

begin mathsize 12px style 1 over straight f equals open parentheses straight mu minus 1 close parentheses open parentheses 1 over straight R plus 1 over straight R close parentheses equals fraction numerator 2 open parentheses straight mu minus 1 close parentheses over denominator straight R end fraction For space each space identical space plano space convex space lens comma space the space focal space length space is fraction numerator 1 over denominator straight f apostrophe end fraction equals fraction numerator open parentheses straight mu minus 1 close parentheses straight R over denominator 2 end fraction equals fraction numerator 1 over denominator 2 straight f end fraction therefore straight f apostrophe equals 2 straight f or space fraction numerator straight f over denominator straight f apostrophe end fraction equals 1 half end style