Question

An object is first placed at a distance 24 cm from the lens and then at a distance of 16 cm from the lens . The magnification of image formed is same in both the cases . What is the focal length of the lens??

Answer 1

hi,

The magnification m in terms of object distance u and focal length f in case of concave mirror is given by

m=f/(f-u)……………(1) For u=-25cm,(according to sign convention u is negative.)

m=f(f+25)…………..(2). For u =-40cm,

4m=f/(f+40)………..(3)

(3)/(2) gives 4=(f+25)/(f+40)

Slmplifying, we get

f -45cm.

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