Question

A double convex lens has two surfaces of equal radii R and refractive index is 15 What is the focal length of the lens?

Answer 1

1f=(n-1) [(1/R1) -(1/R2)]

put the values u will get ur answer

Answer 2

Using lens makers formula,           1/f=(n-1)[(1/R1)-(1/R2)] Here n=15, R1= R and R2= -R Therefore,           1/f=(15-1)[(1/R)-(1/-R)]    =>.  1/f= 14×[2/R]    =>.  1/f= 28/R    =>.  f = R/28