For a concave mirror, the focal point (F) is where parallel rays converge after reflection. When an object is placed exactly at F, the reflected rays become parallel and never converge. Therefore, no real image is formed.

The image is said to be formed at infinity. This is described by the mirror equation: $\frac{1}{f}=\frac{1}{v}+\frac{1}{u}$. When the object distance u = f, the image distance v becomes infinite.

Final Answer: at infinity.