The radii of curvature fo a lens are `+20cm` and `+30cm` . The material of lens has refractive index `mu=(3)//(2)`. Find fcal length of lens in air.

Explanation

Positive sign of radii indicate that with respect to direction of incident rays radii are positive
`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))((1)/(60))`
`f=120cm`