Question

A concave lens of focal length 20cm form an image at a distance of 10cm from the lens.What is the distance of the object from the lens?

Answer 1

Answer:

The object is 20 cm from the lens.

Given:

The concave lens' focal length = 20 cm

Forms an image at a distance of = 10 cm from the lens

To find:

Distance of the object from lens

Solution:

f = –20

v = –10

Remember that image formed in concave lens is virtual.

The lens formula = $\boxed{\rm{ - \frac{1}{u} + \frac{1}{v} = \frac{1}{f}}}$

$\longrightarrow\rm{ - \dfrac{1}{u} - \dfrac{1}{ 10} = - \dfrac{1}{20}}$

$\longrightarrow\rm{ - \dfrac{1}{u} = - \dfrac{1}{20}} + \dfrac{1}{ 10}$

$\longrightarrow\rm{ - \dfrac{1}{u} = - \dfrac{1}{20}} + \dfrac{2}{20}$

$\longrightarrow\rm{ - \dfrac{1}{u} = \dfrac{1}{20}}$

$\longrightarrow\rm{u = - 20}$

Object is placed at 20 cm.

Therefore, the object is 20 cm from the lens.

Answer 2

Answer:

The answer is 20 cm.

The explanation is in the attachment.

Thank you!