Question

With a concave mirror,an object is placed at a distance x1 from principle focus on principle axis .The image formed at a distance x2 from the principle focus.The focus length is???

Answer 1

Focal length is the distance between the centre of the mirror and principal focus....
hope it helps...

Answer 2

The principle focus can be calculated by subtracting x1 from object distance ie. (f=u-xi) or by subtracting x2 from image distance ie.(f=v-x2).