Double-convex lenses are to be manufactured from a glass of refractive index 1.53, with both faces of the same radius of curvature. What is the radius of curvature required if the focal length is to be 20 cm ?

Explanation

`(1)/(f) = (n - 1) ((1)/(R_(1)) - (1)/(R_(2)))`
f = 20 cm , n = 1.55, `R_(1) = R, R_(2) = -R " therefore (1)/(20) = (1.55 - 1) ((1)/(R) + (1)/(R)) = 0.55 xx (2)/(R)`
`R = 0.55 xx 2 x 20 = 22 ` cm