Under what condition, a concave lens can make a real image.

Explanation

Substituting sign of f in the lens formula, we have
`1/v-1/u=1/-f` or `1/v=1/u-1/f`
For real image v should be positive. Therefore, from Eq.(i) we can see that u should be positive
and less than f. Further, u is positive and less than f means a virtual object should lie between O
and `F_1`