An object is placed at the FOCUS of a concave mirror ($u = f$, both negative). The image forms at

1. Mirror formula: $\dfrac{1}{v} + \dfrac{1}{u} = \dfrac{1}{f}$. 2. Substitute $u = f$ (both negative): $\dfrac{1}{v} + \dfrac{1}{f} = \dfrac{1}{f}$. 3. Rearrange: $\dfrac{1}{v} = 0$, so $v \to \infty$. 4. The image forms at INFINITY — meaning the rays emerge PARALLEL after reflection. 5. This is the principle of REVERSIBILITY: an object at the focus of a concave mirror produces a parallel beam (just like a parallel beam from infinity focuses at the focal point — the path of light is reversible). 6. Used in headlights, torches, satellite dishes — source at the focus, parallel output beam. 7. Options A, B, D would require different $u$ values. _Source: syllabus Physics Part 2, Ch 9, §9.2.5 (Mirror formula — object at focus case), p. 4–5._