Final answer:
The formula for the focal length of a concave mirror is f = R / 2, where f is the focal length and R is the radius of curvature, with positive focal length due to the converging nature of a concave mirror.
Step-by-step explanation:
The formula for the focal length (f) in a concave mirror, under the small-angle approximation, is such that the focal length is half of its radius of curvature (R). This can be expressed mathematically as:
f = R / 2
In this case, f represents the focal length, and R represents the mirror's radius of curvature. For a concave mirror, the focal length is positive due to the mirror being converging in nature, and any rays parallel to the principal axis after reflection will converge at a point known as the focal point.