The radii of curvature of both the surfaces of a convex lens are equal. One of the surfaces is made plane by grinding. What is the new focal power of the lens ?

Explanation

Initially for a lens `R_(1)= +R and R_(2) = -R`
Therefore `1//f=(mu-1)(1//R+1//R)=2//R(mu-1)`
When one surface is made plane then `R_(1) = +R and R_(2)= oo`
Therefore, `(1)/(f)=((mu-1))/(R)`
Hence, we find that focal length will become double and power will become half.