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 = -20 cm
• ho = 5 cm
• v = - 15 cm

Find :-

• Distance of object from focal length
• Size of image .

Explanation

Using Formula

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

$\boxed{\underline{\tt{\red{\:m\:=\:\dfrac{h_{i}}{h_{o}}}}}}$

So, First Calculate Distance Object from focal length

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

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

==> 1/u = (3 - 4)/60

==> 1/u = -1/60

==> u = - 60

Now, Calculate image size

==> m = hi/ho = v/u

==> hi = v × ho /u

keep Value,

==> hi = (-15) × 5/(-60)

==> hi = 5/4

==> hi = 1.25 cm

Hence

• Image be diminished & virtual .

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