Question

size of the image of an object by a concave lens of focal length 20 cm is observed to be reduced to 1/3 of its size. Find the distance of the object from the lens.​

Answer 1

Magnification = 1/3

1/3  =  -v/u

u/3  =  v

mirror formula

1/f =  1/v  +  1/u

sub v in formula

1/-20 = 3/u + 1/u

1/-20  =  4/u  

1/u = 1/-80  

u=-80