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?

1 Answer
Jun 1, 2018

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