Question

A converging lens of focal length 5cm is placed at a distance of 20cm from a screen how far from the lens should an object be placed so as to form its real image on the screen?

Answer 1

Hey!

Your answer is in the following attachement.

Answer 2

Answer: -6.67 cm

Step-by-step explanation:

Given: focal length $f=5\ cm$

The image distance $v=20 cm$

Let u be the object distance

By lens formula,

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\\\Rightarrow\frac{1}{u}=\frac{1}{v}-\frac{1}{f}\\\Rightarrow\frac{1}{u}=\frac{1}{20}-\frac{1}{5}\\\Rightarrow\frac{1}{u}=\frac{1-4}{20}\\\Rightarrow\frac{1}{v}=\frac{-3}{20}\\\Rightarrow\ u=-6.666\approx-6.67\ cm$

Hence, the object distance= -6.67 cm