Calculate the focal length of the thin lens shown infigure. The pionts `C_1 and C_2` denote the centres of curvature.

Explanation

As is clear from the ure boththe radii of curvature are positive. Thus, `R_1=+10cm` and `R_2=+20cm.` The focal length is given by
`1/f=(mu-1)(1/R_1-1/R_2)`
`=(1.5-1)(1/(10cm)-1/(20cm))`
`=0.5xx1/(20cm)=1/(40cm)`
`or f=40cm.`