Question

A concave lens has focal length of 12cm. At what distance should the object from the lens be placed so that it froms an image at 8cm from the lens? Also find the magnification by the lens.​

Answer 1

Given:

A concave lens has focal length of 12 cm.

To Find:

The object distance (u) and the magnification by the lens.

Formula:

We know that, 1/v - 1/u = 1/f, and magnification = v/u .

Solution:

Image distance (v) = -8 cm

Focal length (f) = -12 cm

1/v - 1/u = 1/f

or, -1/u = 1/f - 1/v

or, -1/u = 1/-12 - 1/-8

or, -1/u = 1/24

or, u = -24 cm

Magnification = v/u

ﾠﾠﾠﾠﾠﾠﾠﾠﾠﾠﾠﾠ= -8/-24

ﾠﾠﾠﾠﾠﾠﾠﾠﾠﾠﾠﾠ= 1/3

Answer:

The object should be places at a distance of -30 cm from the lens, and the magnification by the lens is 1/3 .