Question

size of image of an object by a concave lens focal length 20 cm is observed to be reduced to 1\3 of its size . find distance of 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