Question

At what distance a boy stand if he observed his image at a distance
of 4 cm in a concave lens if the focal length of that lens is 12 cm.​

Answer 1

Answer:

Boy will stand at a distance of 6 cm

Explanation:

We have given image distance that is v = 4 cm

And focal length of the concave lens f = 12 cm '

We have to find the distance of the boy that is object distance u '

For the lens we know that $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

So $\frac{1}{12}=\frac{1}{4}-\frac{1}{u}$

$\frac{1}{u}=\frac{1}{4}-\frac{1}{12}$

$u=6cm$

So boy will stand at a distance of 6 cm