A convex lens focusses an object 10 cm from it on a screen placed 10 cm away from it. A glass plate `(mu=1.5)` of thickenss 1.5 cm is inserted between the lens and the screen. Where should the object be placed so that its image is again focussed on the screen?

Explanation

`1/f=1/v-1/u=1/(10cm)-(1/(-10cm))=1/(5cm)`
so, the focal length of thelens is 5 cm.The situation with the glass plate inserted is shown in ure. The object is plced at O. The lens would form the image at `I_1` but the glass plate intercepts the rays and forms the image `I ` on the screen.
The shift `I_1I=t(1-1/mu)`
`=(1.5cm)(1-1-1.5)=0.5cm`
Thus, the lens forms the image at a distane 9.5 cm from itself. Using
`=1/v-1/u=1/f`
`1/u=1/v-1/v=1/(9.5cm)-1/(5cm)`
or `u=-10.6cm Thus, the object should be placed at a distace of 10.6 cm from the lens.