Question

convex lens of focal length 0.8 m is placed at a distance of 12 cm from a wall. How far from the lens should an object be placed so as to form it's real image on the wall?

Answer 1

Given:

A convex lens of focal length 0.8 m is placed at a distance of 12 cm from a wall.

To find:

Object distance from lens ?

Calculation:

• Here 12 cm is the image distance where the real image will be formed on the wall.

• And focal length is + 0.8m = 8 cm.

Applying LENS FORMULA:

$\sf \dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u}$

$\sf \implies\dfrac{1}{8} = \dfrac{1}{12} - \dfrac{1}{u}$

$\sf \implies \dfrac{1}{u} = \dfrac{1}{12} - \dfrac{1}{8}$

$\sf \implies \dfrac{1}{u} = \dfrac{2 - 3}{24}$

$\sf \implies \dfrac{1}{u} = \dfrac{ - 1}{24}$

$\sf \implies u = - 24 \: cm$

So, object has to be placed 24 cm from lens.