Lenses are constructed by a material of refractive indices 1'50. The magnitude of the radii of curvature are 20 cm and 30 cm. Find the focal lengths of the possible lenses with the above specifications.

Explanation

There may be four lenses,
`(1)/(f)=(mu-1)((1)/(R_1)-(1)/(R_2))`
(a) `(1)/(f)=((3)/(2)-1)((1)/(20)-(1)/(-30))=((1)/(2))((5)/(60))impliesf=24cm`
(b) `(1)/(f)=((3)/(2)-1)((1)/(20)-(1)/(30))=((1)/(2))((1)/(60))impliesf=120cm`
(c) `(1)/(f)=((3)/(2)-1)((1)/(-20)-(1)/(30))=((1)/(2))(-(5)/(60))`
`f=-24cm`
(d) `(1)/(f)=((3)/(2)-1)((1)/(-20)-(1)/(-30))=((1)/(2))(-(1)/(60))`
`f=-120cm`