Q. 214.8( 5 Votes )

A concave lens has focal length of 20 cm. At what distance from the lens a 5 cm tall object be placed so that it forms an image at 15 cm from the lens? Also calculate the size of the image formed.

Answer :

According to the question;

Object distance = u;

Image distance (v) = -15cm;

Focal length = -20cm

By lens formula;

u = -60cm.

Therefore, object is placed at 60 cm in front of lens.


Height of object h1= 5cm;

Magnification = =

Putting values of v and u

Magnification =

Height of image is 1.25 cm.

Positive sign means image is virtual.

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