A biconvex lens has radii of curvature 20 cm each. If the refractive index of the material of the lens is 1.5, what is its focal length?

Explanation

In a biconvex lens, cenre of curvature of the first surface is on the positive side of the lens and that of the second surface is on the negative side. Thus `R_1=20 cm and R_2=-20 cm `
`We have
`=1/f=(mu-1)(1/R_1-1/R_2)`
`or, 1/f=(1.5-1)(1/(20cm)-1/(-20cm))`
`or f=20cm`