A double convex lens is made of glass of refractive index `1.55` with both faces of same radius of curvature. Find the radius of curvature required, if focal length is `20 cm`.

Explanation

Using the relation,
`1/f=(mu-1)(1/(R_(1))-1/(R_(2))),` we get
`1/20=(1.55-1)[1/R-(1/(-R))]=0.55[1/R+1/R]=0.55xx2/R`
or `R=1.10xx20=22 cm`