The radii of curvature of the faces of a double convex lens are 20 cm and 20 cm. Its focal legnth is also 20 cm. What is the refrctive index of the glass?

Explanation

`R_(1)=+20cm`
`R_(2)=-20cm`
`f=+20cm`
`mu=?`
`1/f=(mu-1)(1/20-1/(-20))`
`1/20=(mu-1)(1/20+1/20)`
`1/20=(mu-1)((1+1)/20)`
`1/20=(mu-1)/20`
`mu-1=1/2`
`mu=1/2+1=3/2`