Find focal lengths of lenses made of glass `(mu=3//2)` and placed in air.

Explanation

(a) `R_1=20cm,R_2=-30cm`
`(1)/(f)=(mu-1)((1)/(R_1)-(1)/(R_2))`
`=((3)/(2)-1)((1)/(20)-(1)/(-30))=((1)/(2))((5)/(6))`
`f=24cm`
(b) `R_1=infty,R_2=-20cm`
`(1)/(f)=((3)/(2)-1)((1)/(infty)-(1)/(-20))=(1)/(40)impliesf=40cm`
(c) `R_1=20cm,R_2=60cm`
`(1)/(f)=((3)/(2)-1)((1)/(20)-(1)/(60))=((1)/(2))((2)/(60))impliesf=60cm`
`R_1=-20,R_2=20cm`
`(1)/(f)=((3)/(2)-1)((1)/(-20)-(1)/(20))=((1)/(2))(-(1)/(10))`
`f=-20cm`