Question

A 2 cm high object is placed 12 cm from a convex lens, perpendicular to the principal axis.The lens forms a real image,whose size is 1.5 cm.Find the power of lens?

Answer 1

You should know that,

`P=1/f`, where `f` is in `meters`.

And you should also know that,

M= image height/object height=`v/u`

Where, `v`= image distance and `u`= object distance

In the question we are given image height, object height and `u`

So first we need to find `v`,

`(1.5cm)/(2cm) = v/2cm`

`=>(2cmxx1.5cm)/(2cm)=v`

`therefore v=1.5cm`

Using mirror's formula,

`1/f=1/v+1/u`

`1/f=1/1.5+1/2`

`1/f=2/3+1/2`

`1/f=(4+3)/6`

`1/f=7/6`

`therefore f=6/7cm`

Convert to metres,

i.e. `6/7-:100=6/700=3/350m`

So to find power,

`P=1/(3/350)D`

`P=350/3D=167 2/3D= 167.667D`

NB: `D` is dioptres