Given;

Radius of Curvature(R) = 50cm;

R = 2f

⇒ 50 = 2f

⇒

Focal length = 25cm.

When an object is kept at the focus of the concave mirror then the image is formed at infinity.

This can be shown as below:

The Mirror formula is given by:

Where, F is the focal length of the mirror

v is the image distance from the pole of the mirror

u is the object distance from the pole of the mirror

On solving for v, we get

If F=u=25 cm, we get

Therefore the object should be placed at 25cm from the pole of the concave mirror to get an image at infinity.