Question

What would be the distance of object from lens?

Real image produced by a convex lens is n times of the object size. If f is the focal length then the distance of object from the lens will be ?
A) nf B) n+1/n *f C) n-1/n*f D) n/n+1 *f
Please mention how/ why! Thanks! :)

Answer 1

The image is real and n times the size of object. So magnification,m =-n

We know m=v/u which means v=-nu

Now use the lens formula

1/v - 1/u = 1/f
-1/nu - 1/u =1/f

which gives u =-nf/(n+1)

Forget the - sign and you get the distance of object from lens.

Answer 2

Why is n - ? And the answer key in my book is B

Answer 3

Oh,yes. The answer will be B)

I made a blunder in the algebra.

It is 1/u = -n/(n+1)f

which gives u=-(n+1)f/n

The minus sign indicates the position of the object. It is a sign convention to use the object distance ,u as a negative number. If we a to report simply the gap b/w mirror and object we use |u| .