A convex lens of foal length 30 cm and a concave lens of focal length 60 cm are placed in contact.If the object is placed 40 cm away from the combination ,, find the position of the image.

Explanation

`f_(1)=+30cm`
`f_(2)=-60cm`
Suppose f be the focal length of combination lenses
`1/f=1/(f_(1))+1/(f_(2))`
`=1/30+1/(-60)=1/30-1/60`
`1/f=(2-1)/60=+1/60`
Distance of object from combination of lenses `=u=-50cm`
disance of image `=v=?`
By lens formula `1/v=1/u=1/f`
`1/v-1/(-40)=1/60`
`1/v+1/40=1/60`
`1/v=1/60-1/40=(2-3)/120=-1/120`
`impliesv=-120cm`