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 8

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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 .

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