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?

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Answer 1 · 2018-06-01T12:22:13

You should know that,

$P = \frac{1}{f}$ $P=1/f$ , where $f$ $f$ is in $m e t e r s$ $meters$ .

And you should also know that,

M= image height/object height= $\frac{v}{u}$ $v/u$

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

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

So first we need to find $v$ $v$ ,

$\frac{1.5 c m}{2 c m} = \frac{v}{2} c m$ $(1.5cm)/(2cm) = v/2cm$

$\Rightarrow \frac{2 c m \times 1.5 c m}{2 c m} = v$ $=>(2cmxx1.5cm)/(2cm)=v$

$therefore v=1.5cm$

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