Question

describe a method to measure the focal length of a concave miror​

Answer 1

Focal Length - The distance between the pole P of the concave mirror and the focus F is the focal length of the concave mirror. Thus, the focal length of a concave mirror can be estimated by obtaining a 'Real image' of a distant object at its focus.

Answer 2

$\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$
where f = focal length
v= image distance from the mirror
u= object distance from the mirror