Question

A concave lens has focal length of 20 cm. At what distance from the lens a 5 cm tall object be placed so that it forms an image at 15 cm from the lens? Also calculate the size of the image formed​

Answer 1

Solution

Given :-

• f = -20cm , v = -15 cm , h = 5 cm

Find :-

• Distance of image from focal length

Explanation

Using Lens Formula

$\dag\boxed{\underline{\tt{\red{\:\dfrac{1}{f}\:=\:\dfrac{1}{v}-\dfrac{1}{u}}}}}$

Keep all values,

==> 1/(-20) = 1/(-15)- 1/u

==> 1/u = 1/20 - 1/15

==> 1/u = (15 - 20)/300

==> 1/u = -5/300

==> 1/u = -1/60

==> u = -60 cm

Now, calculate height of virtual image

• h'/h =v/u

So, keep above Value

==> h'/5 = (-15)/(-60)

==> h'/5 = 1/4

==> h' = 5/4

==> h' = 1.25 cm

Hence

• Distance of objects = 60 cm
• Height of image = 1.25 cm

________________