Question

When an object is placed 8cm from a convex lens, an image is captured on a screen at 4com from the lens. Now the lens is moved along its principal axis while the object and the screen are kept fixed. Where the lens should be moved to obtain another clear?

Answer 1

Object distance and Image distance need to be interchanged.

Explanation:

Common Gaussian form of lens equation is given as
`1/"Object distance"+1/"Image distance"=1/"focal length"`
or
`1/"O"+1/"I"=1/"f"`

Inserting given values we get
`1/8+1/4=1/f`
`=>(1+2)/8=1/f`
`=>f=8/3cm`

Now the lens is being moved, the equation becomes

`1/"O"+1/"I"=3/8`
We see that only other solution is Object distance and Image distance are interchanged.

Hence, if the Object distance is made `=4cm`, clear image would be formed at `8cm`