The focal length of convex tens is `f` and the distance of an object from the principal focus is x. The ratio of the size of the real image to the size of the object is

Explanation

Putting u = - (f + x) and focal length f = -f in
`1/u+1/v=1/f`
We get,
`1/(-(f+x))+1/v=-1/f`
or, `1/v=1/(f+x)-1/f`
= `(f-(f+x))/(f(f+x))`
= `(-x)/(f(f+x))`
`therefore" "v=-((x+f)f)/x`
The required ratio = `(-v)/u=((x+f)f/x)/((x+f))=f/x`