A symmetrical converging convex lens of focal length `10 cm` `&` diverging concave symmetrical lens of focal length `-20 cm` are cut from the middle and perpendicularly and symmetrically to their principal axis. The parts thus obtained are arrenged as shown in the figure. Find the focal length (in `cm`) of this arragement

Answer

80

Explanation

`f_(1)=((+20)(-40))/((20-40)) f_(2)=-40`
`(1)/(f)=(1)/(f_(1))+(1)/(f_(2))-(x)/(f_(1)f_(2))`
`=(1)/(40)+(1)/(-40)-(20)/(40xx(-40))`
`(1)/(f)=(1)/(80)`
`f=80 cm` .