Question

 What is the position of the image by a concave lens when the object is placed at F2?

Answer 1

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\\\\ u = -f\\\\ \frac{1}{v}+\frac{1}{f}=\frac{1}{f}\\\\ \frac{1}{v}=0,\ \ v = \infty \\\\ Hence\ the\ image\ is\ formed\ at\ infinite\ distance..$

When the object is placed at the focus of a lens or a mirror, the image is always formed at the infinity.. It means there is no image formed.  The rays from any point on the object become parallel rays.

Answer 2

The image will form at infinity .