In a concave mirror, the image is formed at infinity when the object is placed exactly at the focal point (F). This is because rays from the object become parallel to the principal axis after reflection, and parallel rays meet at infinity. The mirror equation is:

$\frac{1}{f}=\frac{1}{v}+\frac{1}{u}$

When u = f, then 1/v = 0, so v = ∞. Therefore, the object must be at F.

Final answer: at F.