The correct option is At infinity.
Explanation
Image formation by a spherical mirror is governed by the laws of reflection. The position and nature of the image depend on the object's distance relative to the mirror's pole (P), principal focus (F), and center of curvature (C).
Detailed Analysis
When an object is placed at the principal focus (F) of a concave mirror, the behavior of the light rays is as follows:
- Ray Trajectory: Light rays emanating from a point on the object and striking the mirror reflect in such a way that they become parallel to the principal axis or parallel to each other.
- Intersection: Since parallel rays do not intersect at any finite point, they are assumed to meet at infinity.
- Image Characteristics: Consequently, the image is formed at infinity. This image is real, inverted, and highly magnified.
Comparison with other positions:
- At the pole (P): An object cannot be placed "at" the pole in a practical sense for image formation; however, an object between F and P forms a virtual image behind the mirror.
- Between F and C: The image is formed beyond C.
- Behind the mirror: This occurs when the object is placed between the pole (P) and the focus (F), resulting in a virtual image.
Key Takeaway: In a concave mirror, there is a reciprocal relationship: if the object is at infinity, the image forms at the focus; if the object is at the focus, the image forms at infinity.