An equiconvex lens of focal length `15 cm` is cut into two equal halves in thickness. What is the focal length of each half ?

Explanation

From `(1)/(f) = (mu - 1)((1)/(R_1) - (1)/(R_2))`,
taking `R_1 = R` and `R_2 = - R`
`(1)/(15) = (mu - 1)((1)/( R) + (1)/( R)) = (2)/( R)(mu - 1)`
For each half (plano convex) lens `R_1 = R, R_2 = oo`
`:. (1)/(f')=(mu - 1)((1)/( R)-(1)/(oo)) = (mu - 1)/( R) = (1)/(30)`
`:. f' = 30 cm`.