Question

A convex lens of focal length 10cm is placed at a distance of 12cm from a wall. How far from the lens should an object be placed so,as to form its real image on the wall.

Answer 1

f=10 cm
u=-12cm
v=?
1/f=1/v-1/u
putting the value in this formula and answer is -60cm

Answer 2

by mirror formula:

1/v - 1/u = 1/f 

Sign convention is distance from the center of the lens to be positive in one direction and negative in the other. 

=> 1/12 - 1/u = 1/10 

=> 1/u = 1/12 - 1/10 = - 1/60 

=> the object to be place on side opposite to that of the wall 

at a distance of 60 cm. from the lens.

hope it helps

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