A point source is placed on the axis of a symmetrical convex lens of focal length 20 cm at a distance of 40cm. If the lens is raised by 1cm, by how much will the image be lifted relative to the previous axis?

Explanation

Here, `u=-40cm, f=+20cm`
`:. (1)/(v)-(1)/(u)=(1)/(f)`
or `(1)/(v)=(1)/(20)-(1)/(40)=(2-1)/(40)=(1)/(40)`
`v=40cm`
`m=(v)/(u)=(40)/(-40)=-1`
`:. OP_(1)=mxxAP=(-1)(-1)cm=1cm`
Therefore, the vertical position of `P_(1)` from P is `(1+1)=2cm.` As we know, when mirror is rotated by `theta` , reflected ray is rotated by `2theta`. So, in this case, final image is below PO produced at 10cm.